Improvements to the README item on -Zmiri-track-raw-pointers.

1. The double quotes around <untagged> are changed to backspaces, so <untagged>
   will render correctly in markdown.
2. Clarify that -Zmiri-track-raw-pointers will never accept code that Miri
   would not have accepted without -Zmiri-track-raw-pointers.

Author: Johnathan Van Why

README.md

@@ -252,9 +252,10 @@ environment variable:
 * `-Zmiri-track-raw-pointers` makes Stacked Borrows track a pointer tag even for
   raw pointers. This can make valid code fail to pass the checks, but also can
   help identify latent aliasing issues in code that Miri accepts by default. You
-  can recognize false positives by "<untagged>" occurring in the message -- this
+  can recognize false positives by `<untagged>` occurring in the message -- this
   indicates a pointer that was cast from an integer, so Miri was unable to track
-  this pointer.
+  this pointer. It does not have false negatives; code that works with
+  `-Zmiri-track-raw-pointers` will work without `-Zmiri-track-raw-pointers`.
 
 Some native rustc `-Z` flags are also very relevant for Miri: