Update XML doc to make it clearer that 'compiler.emulatedStack' should no longer be used.

Review at http://gwt-code-reviews.appspot.com/1462804

Review by: robertvawter@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10469 8db76d5a-ed1c-0410-87a9-c151d255dfc7