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
Maniphest Tasks: T10928
Differential Revision: https://secure.phabricator.com/D15861