Document API desc block when released behind a FF

We ask to document on which version a new endpoint is released, but when
using a feature flag is easy to get it wrong.

Here we specify what to write in such case, so that it will be easier to
set the correct GitLab version once we remove the feature flag.

doc/development/api_styleguide.md

@@ -19,7 +19,7 @@ for a good example):
 
 - `desc` for the method summary. You should pass it a block for additional
   details such as:
-  - The GitLab version when the endpoint was added
+  - The GitLab version when the endpoint was added. If it is behind a feature flag, mention that instead: _This feature is gated by the :feature\_flag\_symbol feature flag._
   - If the endpoint is deprecated, and if so, when will it be removed
 
 - `params` for the method params. This acts as description,