vnode_pager_setsize.9: Some clarifications on the manpage

A number of changes:

• Clarifies the locking rules when calling the routine.
• Correct the description regarding the content range to be purged.
• Document the effects on page fault handler.

MFC after: 3 days
MFC with: 86a52e262a6f
Sponsored by: The FreeBSD Foundation
Reviewed by: bcr, kib
Approved by: philip (mentor)
Differential Revision: https://reviews.freebsd.org/D29637