Update references to Terra's classes in docstrings

[adekusar-drl]

What is the expected enhancement?

Review all docstrings in the package and update references to Terra's classes. For instance, in the VQC documentation we reference ``ZZFeatureMap`` that is in qiskit.circuit.library. When the package documentation is generated, the reference becomes just a plain text. Such references should be replaced with cross references, that are replaced in the generated documentations with corresponding links. In this example, in the documentation ``ZZFeatureMap`` to be replaced with `:class:`~qiskit.circuit.library.ZZFeatureMap`. Then in the html documentation this cross reference will become a link to the corresponding class in Terra.