Add class's __init__() docstring too (#3258)
Add class's __init__() docstring too (#3258)

Right now many classes document class constructors and they're arguments
in the docstring for the __init__() methods of the class. However, by
default autodoc does not show these for the classes' documentation and
instead relies on only the class docstring. This commit sets the sphinx
autodoc config option [1] to use both the class and init docstring
together.

[1] https://www.sphinx-doc.org/en/master/usage/extensions/autodoc.html#confval-autoclass_content
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py