Use more precise language to describe what mirroring is

Summary: Fixes T10928. This behavior hasn't changed at all and this is what, e.g., git push --mirror means, and the other behavior isn't possible, but be more explicit about the inherent destructive potential of mirroring.

Test Plan: Read documentation.

Reviewers: chad, sascha-egerer

Reviewed By: sascha-egerer

Subscribers: sascha-egerer

Maniphest Tasks: T10928

Differential Revision: https://secure.phabricator.com/D15861