Document some more socket features

Add some mention of the functions used by protocol implementations,
upcalls, and other general routines.

Not all functionality is documented; in particular:
o the *at() variants, which are useful only for implementing the
  corresponding syscalls.
o soconnect2(), also only used to implement a syscall (socketpair()).
o sockargs(), which is essentually unused and only tangentially
  socket-related.
o selsocket(), which is commented as being present solely for use by
  netncp and netsmb.
o getsockaddr(), which is just a convenience shortcut for copyin().

Reviewed by:	jhb (previous version)
Approved by:	hrs (mentor)
Note that recording is not supported for snd_envy24*

PR:		docs/166755
Approved by:	hrs (mentor)