Add comment about inaccurate error in test.

tests/integration.rs

@@ -2299,6 +2299,8 @@ fn test_hex() {
     assert_eq!(eval("-0xFF"), Ok(Value::Int(-255)));
     assert_eq!(
         eval("0x"),
+        // The "VariableIdentifierNotFound" error is what evalexpr currently returns,
+        // but ideally it would return more specific errors for "illegal" literals.
         Err(EvalexprError::VariableIdentifierNotFound("0x".into()))
     );
 }