Docstrings for `#[new]` method implementations

Discussed in https://github.com/PyO3/pyo3/discussions/4280

^{Originally posted by djc June 23, 2024}
When I write a docstring on a #[new] fn new() method in a #[pymethods] impl block, I would expect this to show up in the help() output for the type. Instead, the help() output seems to end up with a default docstring:

 |  Static methods defined here:
 |
 |  __new__(*args, **kwargs)
 |      Create and return a new object.  See help(type) for accurate signature.

Is there no way in Python's data model to enable a specific help string for this? The docstring for the type ends up making it into the output sort of next to the constructor type signature, so I guess I could use that. Just seems surprising.

I believe that it should be possible to do something similar to what we already do with the text_signature for #[new] to lift the rest of the documentation onto the class. But this needs some experimentation.

Start with the discussion at https://github.com/PyO3/pyo3/discussions/4280 and the existing handling of text_signature for #[new]. Experiment with how documentation from a #[new] method in a #[pymethods] impl appears in Python help() output; done means the method's docstring is surfaced appropriately without regressing the type or constructor signature.