Docs: document how to describe EE-only APIs

The following discussion from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14108#note_181411930 should be addressed:

The problem

Document how to describe EE-only APIs in the docs.

The solution

Clearly describe on the docs style guide how EE-only APIs should be documented. We need this rule so that:

• All API docs are consistent.
• We make it easy to the user to spot what APIs are EE-only.

Since Marcel and Axil are working on merging CE/EE API's docs, I'm assigning this to them.