Documentation/git-filter-branch.txt: Fix description of --commit-filter

Documentation/git-filter-branch.txt: Fix description of --commit-filter

The old description was misleading and logically impossible. It claimed that
the ancestors of the original commit would be re-written to have the multiple
emitted ids as parents. Not only would this modify existing objects, but it
would create a cycle. What this actually does is pass the multiple emitted ids
to the newly-created children to use as parents.

Signed-off-by: Kevin Ballard <kevin@sb.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>