revert: rewrite help summary
New users have a tendency to mistake 'revert' as the command to use to
check out old revisions. They also occasionally mistake revert for a
generalized undo (compare rollback).
This version intentionally aims to avoid mentioning 'earlier' and thus
intentionally no longer alludes to the (secondary) -r behavior (which
in fact is not actually limited to 'earlier').
Instead, we mention checkout state, to convey that we can
restore things to the way they were when checked out.
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