doc: clarify refcount section by referring to folios & pages

Include the rename of subpages_mapcount to _nr_pages_mapped.

Link: https://lkml.kernel.org/r/20230111142915.1001531-4-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>