Documentation/protection-keys: add AArch64 to documentation

As POE support was recently added, update the documentation.

Also note that kernel threads have a default protection key register value.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241001133618.1547996-3-joey.gouly@arm.com


[will: Adjusted wording based on feedback from Kevin]
Signed-off-by: Will Deacon <will@kernel.org>