Class API docs is wrong

Right now, arguments for a class are declared for its constructor (__init__), thus they do not appear at the top of the class docs .
Currently :

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Expected :

To fix this, init arguments must be declared in class docstrings.
More generally, we can avoid arguments specs for the API docs when talking about private functions. Class init being an exception, but we now have to move this to the class docstring.

This is a work in progress in !101 (merged)