Return type for Yield attribute not properly hyperlinked in apidocs

[TheFriendlyCoder]

Describe the bug
Typically when one defines a return value in a method they will use the following syntax:

Returns:
    int:
        some value

When doing so the return type, int in this case, is hyperlinked to the api docs for the appropriate data type (ie: using interphinx).

However, when writing doc strings for generators using the same / similar syntax provided by the Yield attribute, the data type associated with the generator is not appropriately decorated. Taking the previous example code snippet and making a small modification as show below is sufficient to demonstrate the change in behavior:

Yields:
    int:
        some value

The HTML markup produced by this code snippet simply format the data type using custom font formatting based on the theme, but it fails to associate / generate the hyperlink for the API docs for the data type as is done by the Return attribute.

NOTE: I am using the apidocs Sphinx extension to generate the API docs for my projects in case that has any impact on this behavior.

To Reproduce
Steps to reproduce the behavior:

$ git clone git@github.com:TheFriendlyCoder/friendlypins.git
$ cd friendlypins
$ pip install -r requirements.txt
$ tax -e py38-docs
$ open htmldocs/index.html
# Navigate to the docs for the board class (ie: htmldocs/api/friendlypins.board.html#friendlypins.board.Board.pins)

Expected behavior
The data type should be decorated / marked up the same way it appears when using the Return attribute when using the Yield attribute

Your project
https://github.com/TheFriendlyCoder/friendlypins

Screenshots
Example output generated on ReadTheDocs: https://friendlypins.readthedocs.io/en/latest/api/friendlypins.board.html#friendlypins.board.Board.pins

Environment info

• OS: MacOS
• Python version: 3.8.0
• Sphinx version: 3.1.2
• Sphinx extensions: sphinx.ext.autodoc, sphinx.ext.napoleon, sphinx.ext.intersphinx
• Extra tools: any web browser (safari, chrome, firefox, IE)

Additional context

• Data types defined under a Yield attribute aren't properly linked sphinx-contrib/napoleon#25 (I thought this might be a bug with the napoleon extension so I reported it here first, but they recommended I report the issue here instead)

[tristanlatr]

What about introducing a :ytype: field that would specify the type of the yielded values, just as :rtype: ? :)
Then napoleon can use this to link to the type.

[tk0miya]

I posted #8731 to enhance napoleon_preprocess_types for google style docstrings. It allows to convert type definitions in docstrings automatically.