doc: clarify verbose mode

This clarifies that `x` is "verbose mode," and that whitespace becomes
insignificant everywhere, including in character classes. We also add
guidance for how to insert a space: either escape it or use a hex
literal.

Fixes #660

Author: BurntSushi

src/lib.rs

@@ -388,9 +388,13 @@ m     multi-line mode: ^ and $ match begin/end of line
 s     allow . to match \n
 U     swap the meaning of x* and x*?
 u     Unicode support (enabled by default)
-x     ignore whitespace and allow line comments (starting with `#`)
+x     verbose mode, ignores whitespace and allow line comments (starting with `#`)
 </pre>
 
+Note that in verbose mode, whitespace is ignored everywhere, including within
+character classes. To insert whitespace, use its escaped form or a hex literal.
+For example, `\ ` or `\x20` for an ASCII space.
+
 Flags can be toggled within a pattern. Here's an example that matches
 case-insensitively for the first part but case-sensitively for the second part: