socket.2: Cross-reference netintro(4)

netintro(4) is a great manual page that provides a basic introduction to
network facilities, I think it is well worth mentioning in the
socket(2).

I also think we can incorporate this reference somewhere in the text as
well, but I'm not sure, maybe the reference in the SEE ALSO section
would be enough.

Reviewed by: glebius
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D55032