protocol-capabilities.txt: refer multi_ack_detailed back to pack-protocol.txt

pack-protocol.txt explains in detail how multi_ack_detailed works and
what's the difference between no multi_ack, multi_ack and
multi_ack_detailed. No need to repeat here.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/protocol-capabilities.txt

@@ -69,6 +69,12 @@ ends.
 Without multi_ack the client would have sent that c-b-a chain anyway,
 interleaved with S-R-Q.
 
+multi_ack_detailed
+------------------
+This is an extension of multi_ack that permits client to better
+understand the server's in-memory state. See pack-protocol.txt,
+section "Packfile Negotiation" for more information.
+
 thin-pack
 ---------