pmap: add pmap_kextract(9) man page

Add a man page for pmap_kextract(9), with alias to vtophys(9). This man
page is based on pmap_extract(9).

Add it as cross reference in pmap(9), and add comments above the
function implementations.

Co-authored-by: Graham Perrin <grahamperrin@gmail.com>
Co-authored-by: mhorne
Sponsored by: The FreeBSD Foundation
Pull Request: https://github.com/freebsd/freebsd-src/pull/827