document 'quiet' receive-pack capability

This was added in c207e34 (fix push --quiet: add 'quiet'
capability to receive-pack, 2012-01-08) but never
documented.

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

Documentation/technical/protocol-capabilities.txt

@@ -18,7 +18,7 @@ was sent.  Server MUST NOT ignore capabilities that client requested
 and server advertised.  As a consequence of these rules, server MUST
 NOT advertise capabilities it does not understand.
 
-The 'report-status' and 'delete-refs' capabilities are sent and
+The 'report-status', 'delete-refs', and 'quiet' capabilities are sent and
 recognized by the receive-pack (push to server) process.
 
 The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized
@@ -200,3 +200,13 @@ it is capable of accepting a zero-id value as the target
 value of a reference update.  It is not sent back by the client, it
 simply informs the client that it can be sent zero-id values
 to delete references.
+
+quiet
+-----
+
+If the receive-pack server advertises the 'quiet' capability, it is
+capable of silencing human-readable progress output which otherwise may
+be shown when processing the received pack. A send-pack client should
+respond with the 'quiet' capability to suppress server-side progress
+reporting if the local progress reporting is also being suppressed
+(e.g., via `push -q`, or if stderr does not go to a tty).