This updates the documentation for adding EE specific features to
clarify that `prepend` should be on the last line, and how to deal with
code that depends on class methods redefined in EE.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/53919

Signed-off-by: Rémy Coutable <remy@rymai.me>

The private commit email is automatically generated in the format:
id-username@noreply.HOSTNAME

GitLab instance admins are able to change the HOSTNAME portion,
that defaults to Gitlab's hostname, to whatever they prefer.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Label CI/CD has been deprecated (cf. ~CI/CD-DoNotUse ) and its description says to use ~Release or ~Verify

Also:

- Ensures documentation section adheres to linters.
- Makes other minor improvements.

Changes from docs review

Review Apps are now deployed automatically.
Note that if auth fails you might need to stop and redeploy the app.