Update docs for Rugged::Commit#diff.
Update docs for Rugged::Commit#diff.

Acidburn0zzz/rugged,libgit2/rugged,saraid/rugged,tpickett66/rugged,Acidburn0zzz/rugged,saraid/rugged,libgit2/rugged,Acidburn0zzz/rugged,saraid/rugged,tpickett66/rugged,tpickett66/rugged,libgit2/rugged