Clarify behaviour around deleted branches

What does this MR do and why?

Clarify behaviour around deleted branches in docs.

In response to a thread on Slack.

Related to "mirror repository" should be able to prune bra... (#851) (potentially)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

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.