[`flake8-async`] Document thread offloading (`ASYNC240`) (#28008)
## Summary
Document `asyncio.to_thread` and `loop.run_in_executor` as alternatives
for ASYNC240 and make the diagnostic library-neutral.
I noticed that ASYNC240's message and documentation recommended anyio
and trio as alternatives to blocking path operations. This is confusing
in projects where anyio or trio are not used. When making the change I
also noticed some other issues (the rule reports `os.path` _functions_
rather than methods and the docs referenced a nonexistent
`pathlib.Path.dirname` method).
Adds a reference to aiofiles which is a very popular library for async
path operations.
## Test Plan
Built the docs and looked at it.
