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 9f341931
  arm64: mte: Add PROT_MTE support to mmap() and mprotect()
Signed-off-by: NSzabolcs Nagy <szabolcs.nagy@arm.com>
Reviewed-by: NCatalin Marinas <catalin.marinas@arm.com>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20201106101940.5777-1-szabolcs.nagy@arm.comSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/filesystems/proc.rst

@@ -546,6 +546,7 @@ encoded manner. The codes are the following:
     nh    no huge page advise flag
     mg    mergable advise flag
     bt    arm64 BTI guarded page
+    mt    arm64 MTE allocation tags are enabled
     ==    =======================================
 
 Note that there is no guarantee that every flag and associated mnemonic will