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>

Documentation/technical/protocol-capabilities.txt

@@ -21,8 +21,8 @@ NOT advertise capabilities it does not understand.
 The 'report-status' and 'delete-refs' capabilities are sent and
 recognized by the receive-pack (push to server) process.
 
-The 'ofs-delta' capability is sent and recognized by both upload-pack
-and receive-pack protocols.
+The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized
+by both upload-pack and receive-pack protocols.
 
 All other capabilities are only recognized by the upload-pack (fetch
 from server) process.