Added missing docs to DiscoverRunner for keepdb option; refs #20550.

docs/topics/testing/advanced.txt

@@ -309,7 +309,7 @@ behavior. This class defines the ``run_tests()`` entry point, plus a
 selection of other methods that are used to by ``run_tests()`` to set up,
 execute and tear down the test suite.
 
-.. class:: DiscoverRunner(pattern='test*.py', top_level=None, verbosity=1, interactive=True, failfast=True, **kwargs)
+.. class:: DiscoverRunner(pattern='test*.py', top_level=None, verbosity=1, interactive=True, failfast=True, keepdb=False **kwargs)
 
     ``DiscoverRunner`` will search for tests in any file matching ``pattern``.
 
@@ -331,6 +331,10 @@ execute and tear down the test suite.
     If ``failfast`` is ``True``, the test suite will stop running after the
     first test failure is detected.
 
+    If ``keepdb`` is ``True``, the test suite will use the existing database,
+    or create one if necessary. If ``False``, a new database will be created,
+    prompting the user to remove the existing one, if present.
+
     Django may, from time to time, extend the capabilities of the test runner
     by adding new arguments. The ``**kwargs`` declaration allows for this
     expansion. If you subclass ``DiscoverRunner`` or write your own test
@@ -347,6 +351,8 @@ execute and tear down the test suite.
         subclassed test runner to add options to the list of command-line
         options that the :djadmin:`test` command could use.
 
+        The ``keepdb`` argument was added.
+
 Attributes
 ~~~~~~~~~~