Documentation: document /proc api for arm64 MTE vm flags

Documentation: document /proc api for arm64 MTE vm flags

Document that /proc/PID/smaps shows PROT_MTE settings in VmFlags.
Support for this was introduced in

  commit 9f3419315f3cdc41a7318e4d50ba18a592b30c8c
  arm64: mte: Add PROT_MTE support to mmap() and mprotect()

Signed-off-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20201106101940.5777-1-szabolcs.nagy@arm.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>