docs: proc: document ProtectionKey in smaps

The ProtectionKey entry was added in v4.9; back then it was x86-specific,
but it now lives in generic code and applies to all architectures
supporting pkeys (currently x86, power, arm64).

Time to document it: add a paragraph to proc.rst about the ProtectionKey
entry.

[akpm@linux-foundation.org: s/system/hardware/, per review discussion]
[akpm@linux-foundation.org: s/hardware/CPU/]
Link: https://lore.kernel.org/20260407125133.564182-1-kevin.brodsky@arm.com
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Reported-by: Yury Khrustalev <yury.khrustalev@arm.com>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Marc Rutland <mark.rutland@arm.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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