bonding: document the new packets_per_slave option

Add new documentation for the packets_per_slave option available
for balance-rr mode.
Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/bonding.txt

@@ -639,6 +639,15 @@ num_unsol_na
 	are generated by the ipv4 and ipv6 code and the numbers of
 	repetitions cannot be set independently.
 
+packets_per_slave
+
+	Specify the number of packets to transmit through a slave before
+	moving to the next one. When set to 0 then a slave is chosen at
+	random.
+
+	The valid range is 0 - 65535; the default value is 1. This option
+	has effect only in balance-rr mode.
+
 primary
 
 	A string (eth0, eth2, etc) specifying which slave is the