Trait introspection

A class with multiple traits usually has an extremely useless call signature of `__init__`, and is often lacking docstrings. This is understandable, since traits themselves can come with descriptions.

On the other hand, it would be useful to automatically document the traits present in the class in the docstring by e.g. using a decorator, or even fiddle with the `__init__` call signature, since that would enable tools like `sphinx` to produce more complete documentation, and would allow introspection by third party software (unlike the related ipython/ipython#9000).

Start by examining how classes with multiple traits currently expose their __init__ signature and docstrings, and review the related ipython/ipython#9000 discussion. Explore whether a decorator or another introspection mechanism can document the traits for Sphinx and third-party software. Done means trait information is automatically available through the class documentation or signature without relying on IPython-specific behavior.

Written by the indexing model from the issue text.