doc: net: Document POSIX wrappers for socket operations

Document POSIX wrappers for socket operations, by referencing a
corresponsding `zsock_*` function. The documentation will generate a
link where user can read the actual documentation on the socket
operation.

In order to include the POSIX wrappers in to the documentation however,
it's needed to add CONFIG_NET_SOCKETS_POSIX_NAMES to predefined symbol
list in zephyr.doxyfile.in.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>