huge_memory.c: document huge page splitting rules more thoroughly

1. Add information about the behavior of huge page splitting, with
   respect to page/folio refcounts, and gup/pup pins.

2. Update and clarify the existing documentation, to compensate for the
   ravages of time and code change.

Link: https://lkml.kernel.org/r/20240325044452.217463-1-jhubbard@nvidia.com


Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>