Add consistency regarding wording for repository mirroring on branches, tags and commits

What does this MR do and why?

This MR aims to add consistency between the Repository Mirroring documentation and the mirroring repositories settings header.

An Ultimate Customer has contacted support in this ticket (internal only) requesting assistance with configuring mirroring from a specific branch, tag or commit from the source repository. The wording within a project's Setting > Repository provides clarity that this is in regard to mirroring branches, tags, and commits.

This occurs because the entire git repository is mirrored (including branches, tags and commits), when the user provides the git repository URL during setup.

Screenshots or screen recordings

The Repository mirroring page - (current)

The sentence on the Mirroring repositories section of a project Settings > Repository:

How to set up and validate locally

Not applicable, this is a docs change.

MR acceptance checklist

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