net: add documentation to socket.c

stable inclusion
from linux-4.19.196
commit 137e2b34d1c6304b1fa2d55d5232a457bf8b2655

--------------------------------

[ Upstream commit 8a3c245c ]

Adds missing sphinx documentation to the
socket.c's functions. Also fixes some whitespaces.

I also changed the style of older documentation as an
effort to have an uniform documentation style.

Signed-off-by: Pedro Tammela <pctammela@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>