Add asyncio thread safety docs

[bdraco]

Proposed change

Add asyncio thread safety docs.

Since introducing the thread safety checks in 2024.5, I've been watching for matching GitHub issues. Some changes to custom components refer to these checks as "undocumented breaking changes" in Home Assistant. These comments show that developers need help understanding asyncio concepts and thread safety. If they do not realize they have been using the wrong APIs or doing unsafe thread operations and think the check is a breaking change, they may likely create new non-thread-safe operations.

I've also seen some custom components bypass the checks by switching to asyncio.ensure_future or calling loop.create_task directly, which sadly makes the problem worse because the unsafe operation is now harder to find, and users end up reporting crashes that are hard to link to the component.

We should help them determine which API calls should replace the non-thread-safe call to increase the chance the issues will get solved and make developing for Home Assistant easier.

Type of change

• Document existing features within Home Assistant
• Document new or changing features which there is an existing pull request elsewhere
• Spelling or grammatical corrections, or rewording for improved clarity
• Changes to the backend of this documentation
• Removed stale or deprecated documentation

Additional information

• This PR fixes or closes issue: fixes #
• Link to relevant existing code or pull request: Adjust thread safety check messages to point to developer docs core#117392

[frenck]

Looks good to me, thanks, @bdraco 👍

../Frenck

[bdraco]

Thanks. It's already being used by custom component devs.