Documentation RunSQL - add example and emphasis that sqlparse is required

There is no ticket opened for this change, let me know if we need one.

I found that example for RunSQL is very good to have specially as this is all quite new to Django
I think an example that uses `state_operations` would be most helpful. I think passing a string of SQL is rather straightforward. Is the current example even correct? I don't think dropping the table corresponds to `CreateModel` in `state_operations`.