vm_page: update comments and KASSERT()s concerning page allocation

Update the legend describing the arguments to the most commonly used
page allocation functions. Notably, eliminate a reference to a function
that no longer exists; and update to reflect the elimination of
VM_ALLOC_NOOBJ, specifically, VM_ALLOC_WAITOK is no longer a legal
option to vm_page_alloc{,_contig}().

Eliminate a nonsensical KASSERT(). VM_ALLOC_SBUSY is forbidden as an
argument to vm_page_alloc_noobj{,_contig,}_domain() by a KASSERT(), so
having a different KASSERT() that tests for it is nonsensical.

Strengthen other KASSERT()s involving VM_ALLOC_NOBUSY and
VM_ALLOC_NOFREE.

Reviewed by: kib, markj, dougm (an earlier version)
Differential Revision: https://reviews.freebsd.org/D49391