clarify %f documentation

It's natural to expect %f to be an actual file on disk; help avoid that
mistake.

Signed-off-by: Joey Hess <joeyh@joeyh.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -374,6 +374,11 @@ substitution.  For example:
 	smudge = git-p4-filter --smudge %f
 ------------------------
 
+Note that "%f" is the name of the path that is being worked on. Depending
+on the version that is being filtered, the corresponding file on disk may
+not exist, or may have different contents. So, smudge and clean commands
+should not try to access the file on disk, but only act as filters on the
+content provided to them on standard input.
 
 Interaction between checkin/checkout attributes
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^