docs: note that receive-pack knows side-band-64k capability

docs: note that receive-pack knows side-band-64k capability

The protocol-capabilities documentation notes that any
capabilities not explicitly mentioned for receive-pack work
only for upload-pack.

Receive-pack has advertised and understood side-band-64k
since 38a81b4 (receive-pack: Wrap status reports inside
side-band-64k, 2010-02-05), but we do not mention it
explicitly. Let's do so.

Note that receive-pack does not understand side-band, which
was obsolete by that point.

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