Improve the database developer comments and documentation. (#1230)

Author: tim-schilling

example/README.rst

@@ -27,3 +27,10 @@ Before running the example for the first time, you must create a database::
 Then you can use the following command to run the example::
 
     $ python -m django runserver --settings=example.settings
+
+You can change the database used by specifying the ``DJANGO_DATABASE_ENGINE``
+environment variable::
+
+    $ DJANGO_DATABASE_ENGINE=postgresql python -m django migrate --settings=example.settings
+    $ DJANGO_DATABASE_ENGINE=postgresql python -m django runserver --settings=example.settings
+

example/settings.py

@@ -86,6 +86,7 @@
 if os.environ.get("DJANGO_DATABASE_ENGINE", "").lower() == "mysql":
     # % mysql
     # mysql> CREATE DATABASE debug_toolbar;
+    # mysql> CREATE USER 'debug_toolbar'@'localhost';
     # mysql> GRANT ALL PRIVILEGES ON debug_toolbar.* TO 'debug_toolbar'@'localhost';
     DATABASES = {
         "default": {