LUCENE-5211: Better javadocs and error checking of 'format' option in StopFilterFactory, as well as comments in all snowball formated files about specifying format option (merge r1524809)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1524848 13f79535-47bb-0310-9956-ffa450edef68