Support type annotations in `Returns` and `Yields` sections

Currently, when using numpydoc-style docstrings alongside Python type annotation, the types of returns or yields must be specified twice, which is tedious and also error prone. I would like to propose a backward-compatible addition to the docstring specification, which allows returns/yields to be specified without return types:

Returns
-------
return1_name :
    Description of `return1_name`.
return2_name :
    Description of `return2_name`.

Note the addition of the colons to distinguish return names and return types in a backward-compatible way.
Additionally, for a single return value with a Python return type annotation, it would be great if the following would be possible:

Returns
-------
Description of the return

or

Returns
-------
    Description of the return

The same suggestions should also work for the Yields section

Begin with the Returns and Yields docstring parsing behavior and the examples in this issue. Determine how existing typed forms can be distinguished from colon-only and description-only forms, then verify backward compatibility for both sections.