[1.9.x] Fixed #25516 -- Documented that parallel test excution doesn't work with pdb.

Backport of cb15ceb555 from master

docs/ref/django-admin.txt

@@ -1300,6 +1300,10 @@ correctly:
 This feature isn't available on Windows. It doesn't work with the Oracle
 database backend either.
 
+If you want to use :mod:`pdb` while debugging tests, you must disable parallel
+execution (``--parallel=1``). You'll see something like ``bdb.BdbQuit`` if you
+don't.
+
 .. warning::
 
     When test parallelization is enabled and a test fails, Django may be