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>

Documentation/git-filter-branch.txt

@@ -113,7 +113,7 @@ OPTIONS
 	stdin.  The commit id is expected on stdout.
 +
 As a special extension, the commit filter may emit multiple
-commit ids; in that case, ancestors of the original commit will
+commit ids; in that case, the rewritten children of the original commit will
 have all of them as parents.
 +
 You can use the 'map' convenience function in this filter, and other