doc: warn people against --max-pack-size

doc: warn people against --max-pack-size

This option is almost never a good idea, as the resulting repository is
larger and slower (see the new explanations in the docs).

I outlined the potential problems. We could go further and make the
option harder to find (or at least, make the command-line option
descriptions a much more terse "you probably don't want this; see
pack.packsizeLimit for details"). But this seems like a minimal change
that may prevent people from thinking it's more useful than it is.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>