docs: clarify that --encoding can produce invalid sequences

docs: clarify that --encoding can produce invalid sequences

In the common case that the commit encoding matches the
output encoding, we do not touch the buffer at all, which
makes things much more efficient. But it might be unclear to
a consumer that we will pass through bogus sequences.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>