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 <[e-mail]>
Reviewed-by: xu xin <[e-mail]>
Reviewed-by: Zhang Yunkai <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/filesystems/proc.rst

···
  rt_cache      Routing cache
  snmp          SNMP data
  sockstat      Socket statistics
+ softnet_stat  Per-CPU incoming packets queues statistics of online CPUs
  tcp           TCP  sockets
  udp           UDP sockets
  unix          UNIX domain sockets