Documentation: sphinx: Don't parse socket() as identifier reference

With the introduction of Documentation/sphinx/automarkup.py, socket() is
parsed as a reference to the in-kernel definition of socket. Sphinx then
decides that struct socket is a good match, which is usually not
intended, when the syscall is meant instead. This was observed in
Documentation/networking/af_xdp.rst.

Prevent socket() from being misinterpreted by adding it to the Skipfuncs
list in automarkup.py.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>