Suport dynamic symbol docstring adding

PyArrow use [an awkward script](https://github.com/rok/arrow/blob/main/python/scripts/update_stub_docstrings.py) to create docstrings of symbols dynamically created in cython. These symbols come with their generated docstrings that are statically not available. So we first import the full library, then read all docstrings and insert them into annotation stubs.
It would be great to have a more polished tool to do this.
Would a feature like this be considered in `docstring-adder`?

Start by reading PyArrow's python/scripts/update_stub_docstrings.py, linked in the issue, to understand how dynamically created Cython symbols are imported and their docstrings inserted into annotation stubs. Define the scope of a polished docstring-adder feature and consider how its behavior should be tested; done means it can handle this workflow more cleanly.

Written by the indexing model from the issue text.