spec set ops and parens for character classes

This is in support of TR18 1.3 requirements.

S05-regex.pod

@@ -18,7 +18,7 @@ Synopsis 5: Regexes and Rules
     Created: 24 Jun 2002
 
     Last Modified: 8 Feb 2011
-    Version: 140
+    Version: 141
 
 This document summarizes Apocalypse 5, which is about the new regex
 syntax.  We now try to call them I<regex> rather than "regular
@@ -1601,6 +1601,14 @@ These may be combined with pluses and minuses:
 
 =item *
 
+Terms may also be combined using C<|> for set intersection, C<&>
+for set union, and C<^> for symmetric set difference.  Parens may be
+used for grouping.  (Square brackets always quote literal characters
+(including backslashed literal forms), and may not be nested, unlike
+the suggested notation in TR18 section 1.3.)
+
+=item *
+
 The special assertion C<< <.> >> matches any logical grapheme
 (including a Unicode combining character sequences):