add information of incompatibility about Prime.prime? [Bug #7395]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46061 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-05-24

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46062 b2dd03c8-39d4-4d8f-98ff-823fe69b080e