regex support in queryparser needs documented, and called out in CHANGES.txt

Spun off of LUCENE-2604 where everyone agreed this needed done, but no one has done it yet, and rmuir didn't want to leave the issue open...

some issues were pointed out in a recent mailing list thread that definitely seem like they should be addressed before this is officially released...

• queryparsersyntax.xml doesn't mention this feature at all – as major new syntax is should really get it's own section with an example showing the syntax
• queryparsersyntax.xml's section on "Escaping Special Characters" needs to mention that '/' is a special character

Also: Given that Yury encountered some real world situations in which the new syntax caused problems with existing queries, it seems like we should definitely make a note about this possibility more promonient ... i'm not sure if it makes sense in MIGRATE.txt but at a minimum it seems like the existing CHANGES.txt entry should mention it, maybe something like...

* LUCENE-2604: Added RegexpQuery support to QueryParser. Regular expressions
  are now directly supported by the standard queryparser using the syntax... 
     fieldName:/expression/ OR /expression against default field/
  Users who wish to search for literal "/" characters are advised to 
  backslash-escape or quote those characters as needed. 
  (Simon Willnauer, Robert Muir)