net: sockets: Add docstrings for BSD Sockets API

The current idea is that we document zsock_* prefixed symbols, refer
to Open Group POSIX website
(http://pubs.opengroup.org/onlinepubs/9699919799/

) for normative
descriptions, and explicitly mention bare POSIX name of a function
too (so e.g. users could find it via search).

Fixes: #13397

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>