Update documentation for miri-script test changes

Author: RossSmyth

src/tools/miri/CONTRIBUTING.md

@@ -72,14 +72,13 @@ For example:
 
 You can (cross-)run the entire test suite using:
 
-```
-./miri test
-MIRI_TEST_TARGET=i686-unknown-linux-gnu ./miri test
+```sh
+./miri test --target i686-unknown-linux-gnu
 ```
 
 `./miri test FILTER` only runs those tests that contain `FILTER` in their filename (including the
-base directory, e.g. `./miri test fail` will run all compile-fail tests). These filters are passed
-to `cargo test`, so for multiple filters you need to use `./miri test -- FILTER1 FILTER2`.
+base directory, e.g. `./miri test fail` will run all compile-fail tests). Multiple filters 
+are supported: `./miri test FILTER1 FILTER2`.
 
 #### Fine grained logging
 

src/tools/miri/README.md

@@ -464,7 +464,7 @@ by all intended entry points, i.e. `cargo miri` and `./miri {test,run}`):
   setup -- only set this if you do not want to use the automatically created sysroot. When invoking
   `cargo miri setup`, this indicates where the sysroot will be put.
 * `MIRI_TEST_TARGET` (recognized by `./miri {test,run}`) indicates which target
-  architecture to test against.  `miri` and `cargo miri` accept the `--target` flag for the same
+  architecture to test against.  The `--target` flag may be used for the same
   purpose.
 * `MIRI_TEST_THREADS` (recognized by `./miri test`): set the number of threads to use for running tests.
   By default, the number of cores is used.