Documentation/mm: drop pte_bad() descriptions from arch page table helpers

pte_bad() never existed unlike similar helpers at PMU, PUD, and PGD level.
This was added erroneously and hence should be dropped instead.

Link: https://lkml.kernel.org/r/20231114063456.339652-1-anshuman.khandual@arm.com


Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>