docs(cts): note ! prefix for negation of test selection

Author: andyleiserson

xtask/src/main.rs

@@ -29,6 +29,7 @@ Commands:
     --backend <backend>     Specify the backend (metal, dx12, or vulkan). Used
                             to evaluate `fails-if` conditions in the test list.
     --filter <regex>        Filter tests by selector using a regex pattern.
+                            Prefix with '!' to invert (exclude matching tests).
                             Applied after all tests are collected.
 
   run-wasm