Note invalid syntax currently accepted

We currently accept invalid syntax between the parentheses with
`should_panic(..)`.  Let's make a note of that.

Author: traviscross

src/attributes/testing.md

@@ -161,6 +161,9 @@ The `should_panic` attribute has one of the following forms:
   > fn mytest() { panic!("error: some message, and more"); }
   > ```
 
+  > [!NOTE]
+  > `rustc` currently accepts the [MetaListNameValueStr] form with invalid syntax between the parentheses and emits a future-compatibility warning. This may become a hard error in the future.
+
 r[attributes.testing.should_panic.allowed-positions]
 The `should_panic` attribute may only be applied to functions annotated with the `test` attribute.