Document test flag: `generated` (#886)

Resolves gh-885

CONTRIBUTING.md

@@ -137,6 +137,10 @@ This tag is for boolean properties associated with the test.
   `noStrict`
 - **`async`** - defer interpretation of test results until after the invocation
   of the global `$DONE` function
+- **`generated`** - informative flag used to denote test files that were
+  created procedurally using the project's test generation tool; refer to the
+  section titled "Procedurally-generated tests" for more information on this
+  process
 
 #### features
 **features**: [list]

INTERPRETING.md

@@ -284,3 +284,7 @@ following strings:
         print('Error: ' + reason);
       });
   ```
+
+- **`generated`** The test file was created procedurally using the project's
+  test generation tool. This flag is specified for informational purposes only
+  and has no bearing on how the test should be interpreted.