message: Document the new flag
diff: @@ -171,6 +171,11 @@ Several `-Z` flags are relevant for Miri:
  sets this flag per default.
  * `-Zmir-emit-retag` controls whether `Retag` statements are emitted. Miri
  enables this per default because it is needed for validation.
  +* `-Zmiri-track-pointer-tag` aborts interpretation with a backtrace when the
  + given pointer tag is popped from a borrow stack (which is where the tag
  + becomes invalid and any future use of it will error anyway). This helps you
  + in finding out why UB is happening and where in your code would be a good
  + place to look for it.
  Moreover, Miri recognizes some environment variables:
  