Update documentation for separate mirroring username form input

What does this MR do and why?

Followup documentation update for !130334 (merged).

Previously usernames for push and pull mirrors were collected via the Git repository URL field. After !130334 (merged), usernames will be collected separately in a new username field. Not in the Git repository URL

This MR updates the documentation accordingly.

Contributes to #19382 (closed)

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MR acceptance checklist

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Related to #19382 (closed)