Fixed #21116 -- Made usage of manage.py in docs more consistent.

Thanks daniel.quattro at gmail.com for the report.

docs/topics/auth/default.txt

@@ -70,7 +70,7 @@ first time you run it with ``'django.contrib.auth'`` in your
 :setting:`INSTALLED_APPS`. If you need to create a superuser at a later date,
 you can use a command line utility::
 
-    manage.py createsuperuser --username=joe --email=joe@example.com
+    $ python manage.py createsuperuser --username=joe --email=joe@example.com
 
 You will be prompted for a password. After you enter one, the user will be
 created immediately. If you leave off the :djadminopt:`--username` or the

docs/topics/cache.txt

@@ -162,7 +162,7 @@ Database caching
 To use a database table as your cache backend, first create a cache table in
 your database by running this command::
 
-    python manage.py createcachetable [cache_table_name]
+    $ python manage.py createcachetable [cache_table_name]
 
 ...where ``[cache_table_name]`` is the name of the database table to create.
 (This name can be whatever you want, as long as it's a valid table name that's

docs/topics/migrations.txt

@@ -255,7 +255,7 @@ If your app already has models and database tables, and doesn't have migrations
 yet (for example, you created it against a previous Django version), you'll
 need to convert it to use migrations; this is a simple process::
 
-    python manage.py makemigrations yourappname
+    $ python manage.py makemigrations yourappname
 
 This will make a new initial migration for your app. Now, when you run
 :djadmin:`migrate`, Django will detect that you have an initial migration