doc: nrf_modem: update socket options documentation

* Update the documentation to include the newest socket options.
* Display the "supported flags" as a table.
* Align the one-liners better with the @brief from the headers.
* Remove the redundant socket documentation, replace with referal to the
  API documentation.

Signed-off-by: Håvard Vermeer <havard.vermeer@nordicsemi.no>

Author: nordic-hani

nrf_modem/doc/api.rst

@@ -59,11 +59,16 @@ Socket option levels
 
 .. doxygengroup:: nrf_socket_options_levels
 
+
+.. _nrf_socket_generic_options_api:
+
 Generic socket options
 ======================
 
 .. doxygengroup:: nrf_socket_options_sockets
 
+.. _nrf_socket_send_recv_flags_api:
+
 Socket send and recv flags
 ==========================
 
@@ -81,6 +86,8 @@ Socket API
 
 .. doxygengroup:: nrf_socket_api
 
+.. _nrf_socket_tls_api:
+
 TLS socket
 **********