Mention unspace disallowed in regex (except \#)

The current engine disallows unspace in regexex as too ambiguous.
This is fine for actual whitespace, but is counterintuitive when people
try to backslash the # character.  So \# is now specifically allowed
to match a literal # character.

Author: TimToady

S02-bits.pod

@@ -8,8 +8,8 @@ Synopsis 2: Bits and Pieces
 
     Created: 10 Aug 2004
 
-    Last Modified: 2 Sept 2014
-    Version: 285
+    Last Modified: 5 Jan 2015
+    Version: 286
 
 This document summarizes Apocalypse 2, which covers small-scale
 lexical items and typological issues.  (These Synopses also contain
@@ -293,6 +293,17 @@ it is an end-of-line comment, not an embedded comment.  Write:
 
 to mean the other thing.
 
+=head2 Unspace disallowed within regexes
+
+Within a regex, unspace is disallowed as too ambiguous with customary
+backslashing conventions in surrounding cultures.  Hence you must write
+an explicit whitespace match some other way, such as with quotes or with
+a C<\x20> or C<\c32> escape.  On the other hand, while an unspace can start
+with C<\#> in normal code, C<\#> within a regex is specifically allowed,
+and is not taken as unspace, but matches a literal U+0023, NUMBER SIGN.
+(Within a character class, you may also escape whitespace with a backslash;
+the restriction on unspace applies only at the normal pattern-matching level.)
+
 =head2 Optional Whitespace and Exclusions
 
 In general, whitespace is optional in Perl 6 except where it is needed