diff --git a/tornado/testing.py b/tornado/testing.py
index <HASH>..<HASH> 100644
--- a/tornado/testing.py
+++ b/tornado/testing.py
@@ -656,7 +656,9 @@ def main(**kwargs):
 
     This test runner is essentially equivalent to `unittest.main` from
     the standard library, but adds support for tornado-style option
-    parsing and log formatting.
+    parsing and log formatting. It is *not* necessary to use this
+    `main` function to run tests using `AsyncTestCase`; these tests
+    are self-contained and can run with any test runner.
 
     The easiest way to run a test is via the command line::
testing: Clarify docs of testing.main()