Fix some comments.

The comment just below the first one describes how the `impl !Send for
FatalError` makes it impossible to `panic!(FatalError)`.

And the second one should be `panic_any` instead of `panic!`.

Author: nnethercote

compiler/rustc_span/src/fatal_error.rs

@@ -1,13 +1,11 @@
-/// Used as a return value to signify a fatal error occurred. (It is also
-/// used as the argument to panic at the moment, but that will eventually
-/// not be true.)
+/// Used as a return value to signify a fatal error occurred.
 #[derive(Copy, Clone, Debug)]
 #[must_use]
 pub struct FatalError;
 
 pub struct FatalErrorMarker;
 
-// Don't implement Send on FatalError. This makes it impossible to panic!(FatalError).
+// Don't implement Send on FatalError. This makes it impossible to `panic_any!(FatalError)`.
 // We don't want to invoke the panic handler and print a backtrace for fatal errors.
 impl !Send for FatalError {}