Auto merge of rust-lang#2316 - domenicquirl:readme-track-tag, r=RalfJung

Clarify the effect of the `-Zmiri-track-pointer-tag` flag in the README

Edit the README to explicitly say that the `-Zmiri-track-pointer-tag` flag also tracks the creation of tags, not just when they are popped/invalidated.

Related to rust-lang/miri#2308 / Manishearth/triomphe#38.

Author: bors

README.md

@@ -383,8 +383,8 @@ to Miri failing to detect cases of undefined behavior in a program.
   Borrows "protectors". Specifying this argument multiple times does not overwrite the previous
   values, instead it appends its values to the list. Listing an id multiple times has no effect.
 * `-Zmiri-track-pointer-tag=<tag1>,<tag2>,...` shows a backtrace when a given pointer tag
-  is popped from a borrow stack (which is where the tag becomes invalid and any
-  future use of it will error).  This helps you in finding out why UB is
+  is created and when (if ever) it is popped from a borrow stack (which is where the tag becomes invalid 
+  and any future use of it will error).  This helps you in finding out why UB is
   happening and where in your code would be a good place to look for it.
   Specifying this argument multiple times does not overwrite the previous
   values, instead it appends its values to the list. Listing a tag multiple times has no effect.