encoding/xml: add minimal documentation for XML namespace prefixes in Name

Addresses CL feedback from Adam Shannon.

Author: ydnar

src/encoding/xml/read.go

@@ -48,6 +48,10 @@ import (
 //     the given name (and, optionally, namespace) or else Unmarshal
 //     returns an error.
 //
+//   - If the XMLName field contains an XML namespace, it may also
+//     optionally specify a namespace prefix in the form of
+//     "namespace-URL prefix:name".
+//
 //   - If the XML element has an attribute whose name matches a
 //     struct field name with an associated tag containing ",attr" or
 //     the explicit name in a struct field tag of the form "name,attr",

src/encoding/xml/xml.go

@@ -36,7 +36,10 @@ func (e *SyntaxError) Error() string {
 // with a namespace identifier (Space).
 // In tokens returned by [Decoder.Token], the Space identifier
 // is given as a canonical URL, not the short prefix used
-// in the document being parsed.
+// in the document being parsed. If Local is prefixed in
+// the form of "prefix:name", this package will attempt to
+// use the prefix instead of a fully-qualified namespace URL
+// when marshaling.
 type Name struct {
 	Space, Local string
 }