Add documentation for KERNELPACMASK variable being added to the vmcoreinfo.

It indicates the PAC bits mask information of signed kernel pointers if
Armv8.3-A Pointer Authentication feature is present.
Signed-off-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Baoquan He <bhe@redhat.com>
Link: https://lore.kernel.org/r/1589202116-18265-2-git-send-email-amit.kachhap@arm.comSigned-off-by: Will Deacon <will@kernel.org>