net: Document /proc/sys/net/core/netdev_budget

The NAPI poll parameter netdev_budget is not documented in
kernel-docs. Since it may have a substantial effect on at least some
network loads, it should be.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com> 
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/filesystems/proc.txt

@@ -1478,6 +1478,13 @@ of problems on the network like duplicate address or bad checksums. Normally,
 this should be enabled, but if the problem persists the messages can be
 disabled.
 
+netdev_budget
+-------------
+
+Maximum number of packets taken from all interfaces in one polling cycle (NAPI
+poll). In one polling cycle interfaces which are registered to polling are
+probed in a round-robin manner. The limit of packets in one such probe can be
+set per-device via sysfs class/net/<device>/weight .
 
 netdev_max_backlog
 ------------------