Update class_resourceloader.rst

[DevSavvySerge]

Improved the explanation of load_threaded_get behavior and clarified the note about resource lifetime.

[AThousandShips]

Thank you for your contribution!

These changes needs to be done over here, changing the generated files is not allowed, see here

[DevSavvySerge]

Noted, the home page mislead me here.

Edit on GitHub on https://docs.godotengine.org/en/stable/index.html