socket(2): bring documentation up tp date

o Undocument sockets that are no longer supported, or never were.
o Add AF_HYPERV. Note: PF_HYPERV isn't defined, no typo here.
o Point at ip(4) and ip6(4) instead of unwelcoming "not described here".

Reviewed by: gbe, markj
Differential revision: https://reviews.freebsd.org/D36284