Add descriptions for sysctls in kern_mib.c and sysctl.3 which lack them.

r343532 noted the difference between "hw.realmem" and "hw.physmem", which I
was previously unaware of. I discovered that neither sysctl had a
description visible via `sysctl -d', so I found where they were defined and
added suitable descriptions. While in the file, I went ahead and added
descriptions for all the others which lacked them. I also updated sysctl.3
accordingly

Reviewed by: kib, bcr
MFC after: 1 weeks
Sponsored by: Panasas
Differential Revision: https://reviews.freebsd.org/D19007