docs: update THP documentation to clarify sysfs "never" setting

Rather confusingly, setting all Transparent Huge Page sysfs settings to
"never" does not in fact result in THP being globally disabled.

Rather, it results in khugepaged being disabled, but one can still obtain
THP pages using madvise(..., MADV_COLLAPSE).

This is something that has remained poorly documented for some time, and
it is likely the received wisdom of most users of THP that never does, in
fact, mean never.

It is therefore important to highlight, very clearly, that this is not the
case.

[lorenzo.stoakes@oracle.com: update transhuge page to mention MADV_COLLAPSE]
  Link: https://lkml.kernel.org/r/d54d1dfb-f06d-4979-983b-73998f05867e@lucifer.local
Link: https://lkml.kernel.org/r/20250721155530.75944-1-lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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