message: Add tip about `x` flag.
  Fixes
diff: @@ -161,6 +161,10 @@ 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: )`.
  +
  # Example: match multiple regular expressions simultaneously
  This demonstrates how to use a `RegexSet` to match multiple (possibly
  