Include a warning about a multi-threaded pitfall

[gokiburikin]

Describe the project you are working on

A calculation heavy 2D tower defense utilizing multiple threads for performance.

Describe the problem or limitation you are having in your project

Ran into a crash I couldn't understand, singled it out to instancing a specific PackedScene in a thread. Was about to submit a minimal reproducible project when on my last search for an answer I found the culprit: godotengine/godot#20964 (comment)

Switching to Multi-Threaded in project settings fixes the issue, but this is not mentioned in the documentation as a potential pain point and it (probably?) can't be supplied as a warning or error since it's in a thread. To add extra confusion, this only seems to happen if the PackedScene contains a Node that needs to be rendered (Sprite, Control, etc.)

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Adding a notice to the threading page found here: https://docs.godotengine.org/en/stable/tutorials/threads/using_multiple_threads.html

It would have saved me an hour or two and may save others time as well.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

This particular text is not specific to PackedScenes that include renderable Nodes, so it could probably be improved.

If this enhancement will not be used often, can it be worked around with a few lines of script?

Hopefully if people are looking up information on how to use threads they would like to be aware of common pitfalls and how to avoid them. It can be worked around with enough googling to find the answer buried in an issue tracker comment.

Is there a reason why this should be core and not an add-on in the asset library?

This is about improving documentation.