Fixed misreported error codes and elaborated

Documentation mixed up 406 and 405 error codes, this corrects that

Also elaborated on the possible ways you can receive a 405 error code

See spec test: https://gitlab.com/powisj/gitlab-ce/blob/master/spec/requests/api/merge_requests_spec.rb#L848

doc/api/merge_requests.md

@@ -974,10 +974,9 @@ curl --request DELETE --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://git
 
 Merge changes submitted with MR using this API.
 
+If merge request is unable to be accepted (ie: Work in Progress, Closed, Pipeline Pending Completion, or Failed while requiring Success) - you'll get a `405` and the error message 'Method Not Allowed'
 
-If it has some conflicts and can not be merged - you'll get a `405` and the error message 'Branch cannot be merged'
-
-If merge request is already merged or closed - you'll get a `406` and the error message 'Method Not Allowed'
+If it has some conflicts and can not be merged - you'll get a `406` and the error message 'Branch cannot be merged'
 
 If the `sha` parameter is passed and does not match the HEAD of the source - you'll get a `409` and the error message 'SHA does not match HEAD of source branch'