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 made changes by https://github.com/gitlabhq/gitlabhq/commit/3b6f8309f3e74185125ab764ee093f0d75b71fe1