Include the loopback IP in the allowed_hosts header

Document loopback IP usage in allowed_hosts header

If using allowed host and workhorse is listening on TCP, then
allowed_hosts needs the loopback IP to enable internal requests.

Document this configuration and advise administrators of the
implications this may have in their environment.

What does this MR do?

If you plan to use allowed_hosts, and have your workhorse listening on TCP, this is required in order to have keep the internal requests working.

Related issues

#6891 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened