docs: add a note about infinite loops (#3553)

Author: Takashi Matsuo

AUTHORING_GUIDE.md

@@ -287,6 +287,7 @@ example](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/
 * We use parallel processing for tests, so tests should be capable of running in parallel with one another.
 * Use pytest's fixture for resource setup and teardown, instead of
   having them in the test itself.
+* Avoid infinite loops.
 
 ### Arrange, Act, Assert
 
@@ -347,8 +348,43 @@ is expected. Strive to verify the content of the output rather than the syntax.
 For example, the test might verify that a string is included in the output,
 without taking a dependency on where that string occurs in the output.
 
-### Running tests
+### Avoid infinite loops
+
+Never put potential infinite loops in the test code path. A typical
+example is about gRPC's LongRunningOperations. Make sure you pass the
+timeout parameter to the `result()` call.
+
+Good:
+
+```python
+# will raise google.api_core.GoogleAPICallError after 60 seconds
+operation.result(60)
+```
+
+Bad:
+
+```python
+operation.result()  # this could wait forever.
+```
 
+We recommend the timeout parameter to be around the number that gives
+you more than 90% success rate. Don't put too long a timeout.
+
+Now this test is inevitably flaky, so consider marking the test as
+`flaky` as follows:
+
+```python
+
+@pytest.mark.flaky(max_runs=3, min_passes=1)
+def my_flaky_test():
+    # test that involves LRO poling with the timeout
+```
+
+This combination will give you very high success rate with fixed test
+execution time (0.999 success rate and 180 seconds operation wait time
+in the worst case in this example).
+
+### Running tests
 
 Automated testing for samples in `python-docs-samples` is managed by
 [nox](https://nox.readthedocs.io). Nox allows us to run a variety of tests,
@@ -387,7 +423,6 @@ To run a specific test from a specific following:
 nox -s py-3.7 -- snippets_test.py:test_list_blobs
 ```
 
-
 ### Test Environment Setup
 
 Because all tests are system tests that use live resources, running tests