Documentation of `dag_drawer` is unclear on allowed types

[jakelishman]

Talking about qiskit.visualization.dag_visualization:

          What's the reason to have a `not isinstance(dag, DAGCircuit)` path in this function?

Originally posted by @jakelishman in https://github.com/Qiskit/qiskit/pull/12550#discussion_r1713726314

The documentation of dag_drawer implies that only DAGCircuit is allowable, but actually the function is intended to work with DAGDependency as well. We should update the documentation to be accurate.