Move and revise a comment about the relation between the object's paging-
in-progress count and the vnode.  Prior to r188331, we always acquired
the vnode lock before incrementing the object's paging-in-progress count.
Now, we increment it before attempting to acquire the vnode lock with
LK_NOWAIT, but we never sleep acquiring the vnode lock while we have the
count incremented.

In vm_fault()'s loop over the shadow chain, move a comment describing our
invariants to a better place.  Also, add two comments concerning the
relationship between the map and vnode locks.