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.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>