docs: new transfer.advertiseSID option

Document a new config option that allows users to determine whether or
not to advertise their session IDs to remote Git clients and servers.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/transfer.txt

@@ -69,3 +69,7 @@ transfer.unpackLimit::
 	When `fetch.unpackLimit` or `receive.unpackLimit` are
 	not set, the value of this variable is used instead.
 	The default value is 100.
+
+transfer.advertiseSID::
+	Boolean. When true, client and server processes will advertise their
+	unique session IDs to their remote counterpart. Defaults to false.