cat-file doc: document that -e will return some output

The -e option added in 7950571ad7 ("A few more options for
git-cat-file", 2005-12-03) has always errored out with message on
stderr saying that the provided object is malformed, like this:

    $ git cat-file -e malformed; echo $?
    fatal: Not a valid object name malformed
    128

A reader of this documentation may be misled into thinking that

    if ! git cat-file -e "$object" [...]

as opposed to:

    if ! git cat-file -e "$object" 2>/dev/null [...]

is sufficient to implement a truly silent test that checks whether
some arbitrary $object string was both valid, and pointed to an
object that exists.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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