documentation/mm: update pmd_present() in arch_pgtable_helpers.rst

Although pmd_present() might seem to indicate a valid and mapped pmd
entry, in reality it returns true when pmd_page() points to a valid page
in memory , regardless whether the pmd entry is mapped or not.  Andrea
Arcangeli had earlier explained [1] the required semantics for
pmd_present().  This just updates the documentation for pmd_present() as
required.

[1] https://lore.kernel.org/lkml/[email protected]/

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Anshuman Khandual <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Mike Rapoport <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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