doc: note that '\ ' is legal in extended mode

I'm actually not sure when it became legal to escape a ' ' when extended
mode is enabled, but the docs didn't mention it, and they should.

Fixes #523

CHANGELOG.md

@@ -1,3 +1,12 @@
+TBD
+===
+
+Bug fixes:
+
+* [BUG #523](https://github.com/rust-lang/regex/pull/523):
+  Add note to documentation that spaces can be escaped in `x` mode.
+
+
 1.3.7 (2020-04-17)
 ==================
 This release contains a small bug fix that fixes how `regex` forwards crate

src/lib.rs

@@ -152,8 +152,9 @@ assert_eq!(after, "03/14/2012, 01/01/2013 and 07/05/2014");
 ```
 
 If you wish to match against whitespace in this mode, you can still use `\s`,
-`\n`, `\t`, etc. For escaping a single space character, you can use its hex
-character code `\x20` or temporarily disable the `x` flag, e.g., `(?-x: )`.
+`\n`, `\t`, etc. For escaping a single space character, you can escape it
+directly with `\ `, use its hex character code `\x20` or temporarily disable
+the `x` flag, e.g., `(?-x: )`.
 
 # Example: match multiple regular expressions simultaneously