Simplify vm_pageout_init_domain() and add a "big picture" comment.

Stop subtracting 1024/200 from vmd_page_count/200. I cannot see how
such precise accounting can make a difference on modern systems.

Add some explanation of what the page daemon does and how it handles
memory shortages.

Reviewed by: dougm
Discussed with: jeff, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22396