git-filter-branch: Clarify file removal example.

Signed-off-by: NJon Loeliger <jdl@jdl.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -177,6 +177,10 @@ or copyright violation) from all commits:
 git filter-branch --tree-filter 'rm filename' HEAD
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+However, if the file is absent from the tree of some commit,
+a simple `rm filename` will fail for that tree and commit.
+Thus you may instead want to use `rm -f filename` as the script.
+
 A significantly faster version:
 
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