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.
@bbodenmiller I think I will change wiki repo permissions so protected branches on GitLab does not affect wiki repo. In this case Developers can push to wiki repo.