Merge branch 'document-grape-dsl' into 'master'

API methods should be documented using Grape's DSL

## What does this MR do?

Document that we should now document API methods using Grape DSL too.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2397#note_13491048 for the discussion.

See merge request !5594

doc/development/doc_styleguide.md

@@ -244,6 +244,12 @@ In this case:
 Here is a list of must-have items. Use them in the exact order that appears
 on this document. Further explanation is given below.
 
+- Every method must be described using [Grape's DSL](https://github.com/ruby-grape/grape/tree/v0.13.0#describing-methods)
+  (see https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/environments.rb
+  for a good example):
+  - `desc` for the method summary (you can pass it a block for additional details)
+  - `params` for the method params (this acts as description **and** validation
+    of the params)
 - Every method must have the REST API request. For example:
 
     ```