Documentation/git-filter-branch: add a new msg-filter example

There were no example on how to edit commit messages, so add an msg-filter
example.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -240,6 +240,15 @@ committed a merge between P1 and P2, it will be propagated properly
 and all children of the merge will become merge commits with P1,P2
 as their parents instead of the merge commit.
 
+You can rewrite the commit log messages using `--message-filter`.  For
+example, `git-svn-id` strings in a repository created by `git-svn` can
+be removed this way:
+
+-------------------------------------------------------
+git filter-branch --message-filter '
+	sed -e "/^git-svn-id:/d"
+'
+-------------------------------------------------------
 
 To restrict rewriting to only part of the history, specify a revision
 range in addition to the new branch name.  The new branch name will