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
Yes it looks pointless but also it easy to understand and that might be what we want for people discovering this feature.
If we really want to make it more meaning full what about an example to do PostgreSQL Text-Search:

        migrations.RunSQL(
            "ALTER TABLE Posts ADD COLUMN PostText TSVECTOR;"
            "UPDATE Posts SET PostText = to_tsvector(body);"
            "CREATE INDEX PostText_GIN ON Posts USING GIN(PostText);",
            "ALTER TABLE Posts DROP COLUMN PostText; DROP INDEX PostText_GIN"
        )
Or to create a trigger with PostgreSQL:

       migrations.RunSQL(
            "CREATE TRIGGER log_account_update AFTER UPDATE ON table_account"
            "FOR EACH ROW"
            "WHEN (OLD.* IS DISTINCT FROM NEW.*)"
            "EXECUTE PROCEDURE log_account_update();"
        )