API docs don't show init when class string is defined

[dabacon]

Description of the issue

The API docs don't appear to show the doc string for the __init__ method when there is string supplied to the class string. The doc string for the __init__ method is often much more useful.

Here is an example:
https://quantumai.google/reference/python/cirq/ops/ResetChannel

[tanujkhattar]

xref #5151

[tanujkhattar]

Docstring should go in class definition and not in init because init will not show up by default.