Documentation: "rebase <onto> <that>" stays on <that> branch upon exit

Documentation: "rebase <onto> <that>" stays on <that> branch upon exit

This change makes it clearer that the change to the history effected by
executing 'git rebase master' while on 'topic' branch, and by executing
'git rebase master topic' on any branch, will be the same; the implicit
checkout of the second form will remain after the rebase exits.

Signed-off-by: Drew Northup <drew.northup@maine.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>