docs: proc.rst: add softnet_stat to /proc/net table

/proc/net/softnet_stat exists for a long time, but proc.rst miss it.
Softnet_stat shows some statistics of struct softnet_data of online
CPUs. Struct softnet_data manages incoming and output packets
on per-CPU queues. Note that fastroute and cpu_collision in
softnet_stat are obsolete and their value is always 0.

Signed-off-by: Yang Yang <yang.yang29@zte.com.cn>
Reviewed-by: xu xin <xu.xin16@zte.com.cn>
Reviewed-by: Zhang Yunkai <zhang.yunkai@zte.com.cn>
Link: https://lore.kernel.org/r/202212091421536982085@zte.com.cn


Signed-off-by: Jonathan Corbet <corbet@lwn.net>