Clean up the comments and code style in and around vm_pageout_cluster().
In particular, fix factual, grammatical, and spelling errors in various
comments, and remove comments that are out of place in this function.

Reviewed by: kib, markj
MFC after: 3 weeks
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D7410