Document how developers can restrict a feature to a specific subscription level

Description

We are introducing a new kind of check in the codebase, to give access to certain features exclusive to specific subcription levels. For instance, Customer Voice (https://gitlab.com/gitlab-org/gitlab-ee/issues/1675) is EEP/Silver, and should be restricted for users who have access to those plans.

It should be crystal clear for developers in GitLab to know how to implement this check.

Proposal

Create proper documentation for this.

References

• https://gitlab.com/gitlab-org/gitlab-ee/issues/2019
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1510
• https://gitlab.com/gitlab-org/gitlab-ee/issues/1641

cc @rdavila @mydigitalself @DouweM

Also, once this is done, we will evangelize this internally heavily.

After https://gitlab.com/gitlab-org/gitlab-ee/issues/2019, this should be easy :)

changed milestone to %9.2

Moved to %9.2 since https://gitlab.com/gitlab-org/gitlab-ee/issues/2019 is scheduled for %9.2.

changed milestone to %9.5

mentioned in commit 4a3f3c93