Update dagcircuit docstrings for bit class arguments (#2978)
Update dagcircuit docstrings for bit class arguments (#2978)

* Update dagcircuit docstrings for bit class arguments

With the introduction of the Bit classes in #2414 the arguments for
many of the methods in the DAGCircuit class, which often deal with
individual bits, were changed to deal with these new classes instead of
tuples. However, in that process a couple of docstrings were missed and
not updated to reflect the expected types. This commit corrects the
oversight and updates the docstrings for those cases.

Fixes #2820

* Update one more docstring and error message access

QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py