As the following sentence says: "This value is directly passed to :func:`sqlalchemy.create_engine`; refer to its documentation for more information about the allowed syntax.". And the documentation of ``create_engine`` says:

"The string form of the URL is ``dialect+driver://user:password@host/dbname[?key=value..]``, where dialect is a database name such as ``mysql``, ``oracle``, ``postgresql``, etc., and ``driver`` the name of a DBAPI, such as ``psycopg2``, ``pyodbc``, ``cx_oracle``, etc. Alternatively, the URL can be an instance of URL."

So it's not a "general URL" (whatever "general" is supposed to mean here), but a SQLAlchemy URL. The syntax for a SQLite database located on the local file system is introduced in this guide.