message: doc: add a note about the empty regex
  This improvement is a consequence of the Q&A discussion from
  PR
diff: @@ -353,6 +353,9 @@ $ the end of text (or end-of-line with multi-line mode)
  \B not a Unicode word boundary
  </pre>
  +The empty regex is valid and matches the empty string. For example, the empty
  +regex matches `abc` at positions `0`, `1`, `2` and `3`.
  +
  ## Grouping and flags
  <pre class="rust">
  