docs: a note to ourselves about interpreting test failures

Author: nedbat

Diff for: tests/test_process.py

@@ -666,6 +666,7 @@ def assert_tryexecfile_output(self, expected: str, actual: str) -> None:
         """
         # First, is this even credible try_execfile.py output?
         assert '"DATA": "xyzzy"' in actual
+        # If this fails, "+" is actual, and "-" is expected
         assert actual == expected
 
     def test_coverage_run_is_like_python_runme(self) -> None: