Docs - add note about redirects when changing external url

What does this MR do?

I've just come across an issue with a customer where they were trying to change their external_url to something else and had unexpected 302 redirects to their old external url, despite following the regular procedure (change external_url, then gitlab-ctl reconfigure) to change the external url.

We finally figured out that the customer had a home page url defined in their application settings, which was causing the redirect.

This MR addresses this by giving some warning in the docs for users to check this setting before changing the external URL.

Please feel free to change my MR as you see fit!

• Ticket (internal use only) - https://gitlab.zendesk.com/agent/tickets/130019
• Forum - https://forum.gitlab.com/t/backup-with-new-external-url-still-points-to-old-name/14759/7

Related issues

• No related issues

Changing the header title?

• No

Moving docs to a new location?

• No