added reference to Unicode standard

Author: wollmers

S15-unicode.pod

@@ -12,8 +12,8 @@
 =VERSION
 =for table
     Created          2 December 2013
-    Last Modified    17 April 2014
-    Version          6
+    Last Modified    10 October 2014
+    Version          7
 
 This document describes how Unicode and Perl 6 work together. Needless to say,
 it would be good for your chosen reader to support various Unicode characters.
@@ -86,9 +86,11 @@ Perl 6, by default, stores all strings given to it in NFG form, Normalization
 Form Grapheme. It's a Perl 6-invented character representation, designed to deal
 with un-precomposed graphemes properly.
 
-Formally Perl 6 graphemes are defined according to Unicode Grapheme Cluster Boundaries 
-at level "default extended" (in contrast to "taylored" or lower levels). This is the same 
-as the Perl 5 character class
+Formally Perl 6 graphemes are defined exactly according to Unicode Grapheme Cluster Boundaries 
+at level "extended" (in contrast to "taylored" or "legacy"), 
+see Unicode Standard Annex #29 UNICODE TEXT SEGMENTATION 
+L<http://www.unicode.org/reports/tr29/tr29-25.html#Grapheme_Cluster_Boundaries|3 Grapheme Cluster Boundaries>>. 
+This is the same as the Perl 5 character class
 
   \X   Match Unicode "eXtended grapheme cluster"