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 saw in https://github.com/gitlabhq/gitlabhq/commit/416018062131e048fd5e97a3d399bb20e771e965#commitcomment-7981617 you linked https://github.com/gitlabhq/gitlabhq/blob/7-1-stable/lib/api/internal.rb#L16-L22 but that section of code strips .wiki from the pathname if I understand correctly, not do anything with protected branches.