Update docs to put prepend as the last line

What does this MR do?

This updates the documentation for our use of prepend for EE modules, stating that prepend should be the last line as well as adding some examples on how to better deal with class methods redefined in EE. See https://gitlab.com/gitlab-org/gitlab-ce/issues/53919 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22987 for more information.

Related issues

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

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs

Review checklist

• Your team's review (required)
• Technical writer's review (required)