Add comments to explain invalid fixture

Author: Ryu0118

Tests/Fixtures/invalid_configs_value_non_dict.yml

@@ -1,5 +1,8 @@
 name: InvalidConfigsNonDict
 
+# This fixture is used to test the validation that ensures all values under `settings.configs`
+# are mappings. `invalid_key0` and `invalid_key1` are not mapping,
+# which should cause a SpecParsingError.invalidConfigsMappingFormat to be thrown.
 settings:
   configs:
     invalid_key0: value0