Document flags of vm_page allocation functions.

Reviewed by:	alc


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276054 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Do not clear flag that vm_page_alloc() doesn't support.

Submitted by:	kib


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276055 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f