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.
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
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