docs/mm: describe set_memory() and set_direct_map() APIs

The set_memory() and set_direct_map() APIs change permissions of existing
kernel mappings, but their semantics are only described by the code

docs/mm: describe set_memory() and set_direct_map() APIs

The set_memory() and set_direct_map() APIs change permissions of existing
kernel mappings, but their semantics are only described by the code, and
that code differs from architecture to architecture.

Add Documentation/mm/kernel-page-tables.rst that briefly describes what
the kernel page tables consist of, defines the semantics both APIs have in
common, including the parts that are easy to get wrong, and lists the
differences between the architecture implementations.

Add kernel-doc comments for the generic set_memory() and set_direct_map()
stubs and link them into Documentation/core-api/mm-api.rst.

[rppt@kernel.org: fix phrasing, per Kevin]
Link: https://lore.kernel.org/arJNn2QD_pY6e14R@kernel.org
Link: https://lore.kernel.org/20260919-set-memory-docs-v2-1-a2a4b3657690@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Kevin Brodsky <kevin.brodsky@arm.com>
Assisted-by: copilot:claude-opus
Cc: "David Hildenbrand (arm)" <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Shuah khan <skhan@linuxfoundation.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: "Vlastimil Babka (SUSE)" <vbabka@kernel.org>

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