Add a note about tool script inheritance

[bmolyneaux]

Related proposal: godotengine/godot-proposals#754

[dalexeev]

• Maybe we will make it generate an error, see GDScript: Add warning if non-@tool class extends @tool class godot#78178.

[bmolyneaux]

Thanks for making the pull request!

The proposal godotengine/godot-proposals#754 has been open for over 3 years. This subtlety has been undocumented for a long time. I expect it will still be some time before the discussion around this proposal is resolved and your pull request is merged. A warning message would be helpful, but I don't think it negates the need to document this behavior.

[mhilbrunner]

Merging this for now, as noted above, even if fixed in a future version, we can then update the docs for that version. :) Thanks for the PR!