mm: add docs for per-order mTHP split counters

This commit introduces documentation for mTHP split counters in
transhuge.rst.

[ioworker0@gmail.com: improve the doc as suggested by Ryan]
  Link: https://lkml.kernel.org/r/20240704012905.42971-3-ioworker0@gmail.com
[ioworker0@gmail.com: tweak Documentation/admin-guide/mm/transhuge.rst]
  Link: https://lkml.kernel.org/r/20240707013659.1151-1-ioworker0@gmail.com
Link: https://lkml.kernel.org/r/20240628130750.73097-3-ioworker0@gmail.com


Signed-off-by: Mingzhe Yang <mingzhe.yang@ly.com>
Signed-off-by: Lance Yang <ioworker0@gmail.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Bang Li <libang.li@antgroup.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>