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.
@randx I agree that wiki permissions should be handled differently. I was just creating this in case you guys don't have a chance to fix before the next release. Please also see https://github.com/gitlabhq/gitlabhq/issues/7916 (currently wiki is being protected from push edits but not web edits).