pack-objects doc: treat output filename as opaque

After 1190a1a (pack-objects: name pack files after trailer hash,
2013-12-05), the SHA-1 used to determine the filename is calculated
differently.  Update the documentation to not guarantee anything more
than that the SHA-1 depends on the pack content somehow.

Hopefully this will discourage readers from depending on the old or
the new calculation.

Reported-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>