add note about wiki protection & improve wording

Add a note about how the wiki branches are protected using these same settings as can be seen at https://github.com/gitlabhq/gitlabhq/blob/master/lib/api/internal.rb#L18-L24. Also improves wording of protected branch details.
I think it may be a mistake from our side. According to permissions table we  want developers to manage wiki. And protecting master branch for wiki repo wont allow developer to do a push. So we might want to provide different permission check for wiki repo. I will think about it and give some feedback later