Tidy up. Mostly, remove or replace stale comments. Most of the comments
in this file actually described the operation of the swap pager, not the
default pager. Given that this is the wrong place to discuss the
implementation of the swap pager, it shouldn't come as a surprise that as
the swap pager evolved these comments became increasingly stale. In
addition, apply some style fixes, like modernizing a few remaining old-
style function definitions.

Reviewed by: kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D8781