Documentation: add description for net.core.gro_normal_batch

Describe it in admin-guide/sysctl/net.rst like other Network core options.
Users need to know gro_normal_batch for performance tuning.

Fixes: 323ebb61 ("net: use listified RX for handling GRO_NORMAL skbs")
Reported-by: Prijesh Patel <prpatel@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://lore.kernel.org/r/acf8a2c03b91bcde11f67ff89b6050089c0712a3.1652888963.git.lucien.xin@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/admin-guide/sysctl/net.rst

@@ -374,6 +374,15 @@ option is set to SOCK_TXREHASH_DEFAULT (i. e. not overridden by setsockopt).
 If set to 1 (default), hash rethink is performed on listening socket.
 If set to 0, hash rethink is not performed.
 
+gro_normal_batch
+----------------
+
+Maximum number of the segments to batch up on output of GRO. When a packet
+exits GRO, either as a coalesced superframe or as an original packet which
+GRO has decided not to coalesce, it is placed on a per-NAPI list. This
+list is then passed to the stack when the number of segments reaches the
+gro_normal_batch limit.
+
 2. /proc/sys/net/unix - Parameters for Unix domain sockets
 ----------------------------------------------------------