Documentation: Refer to git-rebase(1) to warn against rewriting

This points readers at the "Recovering from upstream rebase" warning
in git-rebase(1) when we talk about rewriting published history in the
'reset', 'commit --amend', and 'filter-branch' documentation.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>