Add unidiff option to merge_request_diffs documentation

Author: travisdock

lib/gitlab/client/merge_requests.rb

@@ -367,6 +367,7 @@ def delete_merge_request(project, merge_request_id)
     # @param  [Integer] id The ID of a merge request.
     # @option options [Integer] :page The page number.
     # @option options [Integer] :per_page The number of results per page.
+    # @option options [Boolean] :unidiff Present diffs in the unified diff format.
     # @return [Gitlab::ObjectifiedHash] A list of the merge request diffs.
     def merge_request_diffs(project, merge_request_id, options = {})
       get("/projects/#{url_encode project}/merge_requests/#{merge_request_id}/diffs", query: options)