Add 'rm -f' equivalent to 'git rm' example of filter-branch --index-f…

…ilter

Signed-off-by: Jacob Helwig <jacob.helwig@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -212,6 +212,11 @@ git filter-branch --index-filter 'git rm --cached filename' HEAD
 
 Now, you will get the rewritten history saved in HEAD.
 
+As with using `rm filename`, `git rm --cached filename` will fail
+if the file is absent from the tree of a commit.  If it is not important
+whether the file is already absent from the tree, you can use
+`git rm --cached --ignore-unmatch filename` instead.
+
 To rewrite the repository to look as if `foodir/` had been its project
 root, and discard all other history: