Adds more docstring highlights missing from #20486.
[PEP257](https://peps.python.org/pep-0257/) defines attribute docstrings
as
> String literals occurring immediately after a simple assignment at the
top level of a module, class, or __init__ method are called “attribute
docstrings”.
This PR adds `@string.doc` for such cases.
Before:

After:

Release Notes:
- Added Python syntax highlighting for attribute docstrings.
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Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>