Summary

Expands the docstring of ObservablesArray.tolist to make it clear it might return a scalar in the case the observables array is of dimension 0.

Fixes #13402.

Details and comments

ObservablesArray.tolist is based on numpy's tolist method which returns a list with nesting level equal to the dimension of the numpy array being converted. When an observable array is initialized using a single observable (as opposed to a list containing a single observable) the internal representation of the array will be as a dimension 0 numpy array, resulting on a somewhat confusing return value:

>>> from qiskit.primitives.containers.observables_array import ObservablesArray
>>> oa = ObservablesArray.coerce(["Z"])
>>> print(type(oa.tolist()))
<class 'list'>
>>> oa = ObservablesArray.coerce("Z")
>>> print(type(oa.tolist()))
<class 'dict'>

This PR clarifies the issue in the method's docstring.