Add note about Go Unicode character classes

As proposed on golang-nuts:
  https://groups.google.com/g/golang-nuts/c/M3lmSUptExQ/m/hRySV9GsCAAJ

See also:
  https://go-review.googlesource.com/c/go/+/264678

Change-Id: I028dd1abccfd5372f31e21c3453fc9a40cc343ad
Reviewed-on: https://code-review.googlesource.com/c/re2/+/58030
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/doc/mksyntaxgo b/doc/mksyntaxgo
index caad9b6..3df5559 100755
--- a/doc/mksyntaxgo
+++ b/doc/mksyntaxgo
@@ -33,6 +33,7 @@
 
 .
 $a
+Unicode character classes are those in unicode.Categories and unicode.Scripts.
 */
 package syntax
 .