Add documentation on new admin global ip setting

What does this MR do and why?

Describe in detail what your merge request does and why.

Updates the documentation to add some basic information on the new "Globally allowed IP ranges" field in the admin. This is a follow-up from !87579 (merged).

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/362700

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.