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

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/20181017020930.GN30832@redhat.com/

Link: https://lkml.kernel.org/r/20221123051319.1312582-1-anshuman.khandual@arm.com
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Rapoport <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>