net: add missing sk_max_pacing_rate doc

Warning(include/net/sock.h:411): No description found for parameter
'sk_max_pacing_rate'

Lets please "make htmldocs" and kbuild bot.

Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Change-Id: I0a7952b50cf3df1dc80011ebef685d4dfd0f6dc6
Signed-off-by: Kevin F. Haggerty <haggertk@lineageos.org>