Clean up documentation about test concurrency

docs/en_us/internal/testing.rst

@@ -208,10 +208,16 @@ To run a single test format the command like this.
 
     paver test_system -t lms/djangoapps/courseware/tests/tests.py:ActivateLoginTest.test_activate_login
 
-The ``lms`` suite of tests runs concurrently, and with randomized order, by default.
-You can override these by using ``--no-randomize`` to disable randomization,
-and ``--processes=N`` to control how many tests will run concurrently (``0`` will
-disable concurrency). For example:
+The ``lms`` suite of tests runs with randomized order, by default.
+You can override these by using ``--no-randomize`` to disable randomization.
+
+You can also enable test concurrency with the ``--processes=N`` flag (where ``N``
+is the number of processes to run tests with, and ``-1`` means one process per
+available core). Note, however, that when running concurrently, breakpoints may
+not work correctly, and you will not be able to run single test methods (only
+single test classes).
+
+For example:
 
 ::
     # This will run all tests in the order that they appear in their files, serially