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'm not seeing a difference between https://github.com/gitlabhq/gitlabhq/blob/master/lib/api/internal.rb#L18-L24 and https://github.com/gitlabhq/gitlabhq/blob/7-1-stable/lib/api/internal.rb#L16-L22. On 7.1 it definitely protects the wiki branches the same way as the code branches. 

From https://github.com/gitlabhq/gitlabhq/blob/master/lib/api/internal.rb#L20: 
> This applies the correct project permissions to the wiki repository as well.