Update heap_page_prune() free space map comments.

Update heap_page_prune() free space map comments.

It is up to the heap_page_prune() caller to decide what to do about
updating the FSM for a page following pruning.  Update old comments that
address what we might want to do as if it was the responsibility of
heap_page_prune() itself.  heap_page_prune() doesn't have enough
high-level context to make a sensible choice.