Documented that the parallel test runner doesn't work on Windows.

docs/ref/django-admin.txt

@@ -1289,7 +1289,8 @@ correctly:
 
     $ pip install tblib
 
-This feature isn't available on Oracle.
+This feature isn't available on Windows. It doesn't work with the Oracle
+database backend either.
 
 .. warning::
 

docs/releases/1.9.txt

@@ -135,9 +135,10 @@ Each process gets its own database. You must ensure that different test cases
 don't access the same resources. For instance, test cases that touch the
 filesystem should create a temporary directory for their own use.
 
-This option is enabled by default for Django's own test suite on database
-backends that support it (this is all the built-in backends except for
-Oracle).
+This option is enabled by default for Django's own test suite provided:
+
+- the OS supports it (all but Windows)
+- the database backend supports it (all the built-in backends but Oracle)
 
 Minor features
 ~~~~~~~~~~~~~~

tests/runtests.py

@@ -236,8 +236,11 @@ def teardown(state):
 
 def actual_test_processes(parallel):
     if parallel == 0:
+        # On Python 3.4+: if multiprocessing.get_start_method() != 'fork':
+        if not hasattr(os, 'fork'):
+            return 1
         # This doesn't work before django.setup() on some databases.
-        if all(conn.features.can_clone_databases for conn in connections.all()):
+        elif all(conn.features.can_clone_databases for conn in connections.all()):
             return default_test_processes()
         else:
             return 1