spec: retitle section on "Assignments" to "Assignment statements"

This permits a clear distinction between an individual assignment
and an assignment statement which may assign more than one value.
It also makes this section title consistent with all other section
titles about statements. Adjust internal links and prose where
appropriate. (Note that the spec already referred to assignment
statements in a couple of places, even before this change.)

Add an introductory paragraph to the section on assignment statements.

Preparation for adding a section on value vs reference types
(issue #5083).

Change-Id: Ie140ac296e653c67da2a5a203b63352b3dc4f9f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/413615
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

Author: griesemer

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of June 21, 2022",
+	"Subtitle": "Version of June 29, 2022",
 	"Path": "/ref/spec"
 }-->
 
@@ -263,7 +263,7 @@ <h3 id="Operators_and_punctuation">Operators and punctuation</h3>
 
 <p>
 The following character sequences represent <a href="#Operators">operators</a>
-(including <a href="#Assignments">assignment operators</a>) and punctuation:
+(including <a href="#Assignment_statements">assignment operators</a>) and punctuation:
 </p>
 <pre class="grammar">
 +    &amp;     +=    &amp;=     &amp;&amp;    ==    !=    (    )
@@ -676,7 +676,7 @@ <h2 id="Constants">Constants</h2>
 A constant may be given a type explicitly by a <a href="#Constant_declarations">constant declaration</a>
 or <a href="#Conversions">conversion</a>, or implicitly when used in a
 <a href="#Variable_declarations">variable declaration</a> or an
-<a href="#Assignments">assignment</a> or as an
+<a href="#Assignment_statements">assignment statement</a> or as an
 operand in an <a href="#Expressions">expression</a>.
 It is an error if the constant value
 cannot be <a href="#Representability">represented</a> as a value of the respective type.
@@ -780,7 +780,7 @@ <h2 id="Variables">Variables</h2>
 <p>
 A variable's value is retrieved by referring to the variable in an
 <a href="#Expressions">expression</a>; it is the most recent value
-<a href="#Assignments">assigned</a> to the variable.
+<a href="#Assignment_statements">assigned</a> to the variable.
 If a variable has not yet been assigned a value, its value is the
 <a href="#The_zero_value">zero value</a> for its type.
 </p>
@@ -1591,7 +1591,7 @@ <h3 id="Map_types">Map types</h3>
 For a map <code>m</code>, it can be discovered using the
 built-in function <a href="#Length_and_capacity"><code>len</code></a>
 and may change during execution. Elements may be added during execution
-using <a href="#Assignments">assignments</a> and retrieved with
+using <a href="#Assignment_statements">assignments</a> and retrieved with
 <a href="#Index_expressions">index expressions</a>; they may be removed with the
 <a href="#Deletion_of_map_elements"><code>delete</code></a> built-in function.
 </p>
@@ -1634,7 +1634,7 @@ <h3 id="Channel_types">Channel types</h3>
 <i>send</i> or <i>receive</i>. If a direction is given, the channel is <i>directional</i>,
 otherwise it is <i>bidirectional</i>.
 A channel may be constrained only to send or only to receive by
-<a href="#Assignments">assignment</a> or
+<a href="#Assignment_statements">assignment</a> or
 explicit <a href="#Conversions">conversion</a>.
 </p>
 
@@ -2241,7 +2241,7 @@ <h3 id="Blank_identifier">Blank identifier</h3>
 The <i>blank identifier</i> is represented by the underscore character <code>_</code>.
 It serves as an anonymous placeholder instead of a regular (non-blank)
 identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>,
-as an <a href="#Operands">operand</a>, and in <a href="#Assignments">assignments</a>.
+as an <a href="#Operands">operand</a>, and in <a href="#Assignment_statements">assignment statements</a>.
 </p>
 
 
@@ -2748,7 +2748,7 @@ <h3 id="Variable_declarations">Variable declarations</h3>
 
 <p>
 If a list of expressions is given, the variables are initialized
-with the expressions following the rules for <a href="#Assignments">assignments</a>.
+with the expressions following the rules for <a href="#Assignment_statements">assignment statements</a>.
 Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>.
 </p>
 
@@ -3011,7 +3011,7 @@ <h3 id="Operands">Operands</h3>
 
 <p>
 The <a href="#Blank_identifier">blank identifier</a> may appear as an
-operand only on the left-hand side of an <a href="#Assignments">assignment</a>.
+operand only on the left-hand side of an <a href="#Assignment_statements">assignment statement</a>.
 </p>
 
 <p>
@@ -3821,7 +3821,7 @@ <h3 id="Index_expressions">Index expressions</h3>
 
 <p>
 An index expression on a map <code>a</code> of type <code>map[K]V</code>
-used in an <a href="#Assignments">assignment</a> or initialization of the special form
+used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
 </p>
 
 <pre>
@@ -4037,7 +4037,7 @@ <h3 id="Type_assertions">Type assertions</h3>
 </pre>
 
 <p>
-A type assertion used in an <a href="#Assignments">assignment</a> or initialization of the special form
+A type assertion used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
 </p>
 
 <pre>
@@ -5194,7 +5194,7 @@ <h3 id="Receive_operator">Receive operator</h3>
 </pre>
 
 <p>
-A receive expression used in an <a href="#Assignments">assignment</a> or initialization of the special form
+A receive expression used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
 </p>
 
 <pre>
@@ -5942,7 +5942,7 @@ <h3 id="IncDec_statements">IncDec statements</h3>
 </pre>
 
 <p>
-The following <a href="#Assignments">assignment statements</a> are semantically
+The following <a href="#Assignment_statements">assignment statements</a> are semantically
 equivalent:
 </p>
 
@@ -5953,7 +5953,14 @@ <h3 id="IncDec_statements">IncDec statements</h3>
 </pre>
 
 
-<h3 id="Assignments">Assignments</h3>
+<h3 id="Assignment_statements">Assignment statements</h3>
+
+<p>
+An <i>assignment</i> replaces the current value stored in a <a href="#Variables">variable</a>
+with a new value specified by an <a href="#Expressions">expression</a>.
+An assignment statement may assign a single value to a single variable, or multiple values to a
+matching number of variables.
+</p>
 
 <pre class="ebnf">
 Assignment = ExpressionList assign_op ExpressionList .
@@ -6522,7 +6529,7 @@ <h4 id="For_range">For statements with <code>range</code> clause</h4>
 
 <p>
 The iteration values are assigned to the respective
-iteration variables as in an <a href="#Assignments">assignment statement</a>.
+iteration variables as in an <a href="#Assignment_statements">assignment statement</a>.
 </p>
 
 <p>