[1.6.x] Fixed #21068 -- Added some docs for DiscoverRunner

Thanks jcd.

Backport of e4b012feeb from master

docs/topics/testing/advanced.txt

@@ -300,11 +300,13 @@ Defining a test runner
 
 .. currentmodule:: django.test.runner
 
+.. versionadded:: 1.6
+
 A test runner is a class defining a ``run_tests()`` method. Django ships
-with a ``DiscoverRunner`` class that defines the default Django
+with a ``DiscoverRunner`` class that defines the default Django testing
-testing behavior. This class defines the ``run_tests()`` entry point,
+behavior. This class defines the ``run_tests()`` entry point, plus a
-plus a selection of other methods that are used to by ``run_tests()`` to
+selection of other methods that are used to by ``run_tests()`` to set up,
-set up, execute and tear down the test suite.
+execute and tear down the test suite.
 
 .. class:: DiscoverRunner(pattern='test*.py', top_level=None, verbosity=1, interactive=True, failfast=True, **kwargs)
 
@@ -341,6 +343,13 @@ set up, execute and tear down the test suite.
 Attributes
 ~~~~~~~~~~
 
+.. attribute:: DiscoverRunner.test_loader
+
+    This is the class that loads tests, whether from TestCases or modules or
+    otherwise and bundles them into test suites for the runner to execute.
+    By default it is set to ``unittest.defaultTestLoader``. You can override
+    this attribute if your tests are going to be loaded in unusual ways.
+
 .. attribute:: DiscoverRunner.option_list
 
     This is the tuple of ``optparse`` options which will be fed into the