[S05] Expand information on <|rule> syntax.

Wouldn't want someone thinking that was an exhaustive list, now would
we? :)

S05-regex.pod

@@ -1967,7 +1967,9 @@ for free, which fails if the current rule would match again at this location.
 
 =item *
 
-A leading C<|> indicates some kind of a zero-width boundary.
+A leading C<|> indicates some kind of a zero-width boundary. You can refer to
+backslash sequences with this syntax; C<< <|h> >> will match between a C<\h> and
+a C<\H>, for instance. Some examples:
 
     <|w> word boundary
     <|g> grapheme boundary (always matches in grapheme mode)