Clarify the global registration of named_classes

[thibault]

Hi,

This is my first constribution to the great Godot's documentation, I hope it will be useful.

While reading the doc about gdscript classes, I found the usage of the keyword class_name confusing. Notably, the fact that a named class becomes available globally does not seem intuitive to me, as it's not clearly stated.

Seing that gdscript draws some inspiration from Python, as a newcomer it seems reasonable to think that any class must be imported before being used, and that was my first assumption.

Here is a clarification suggestion for the documentation. Feel free to tell me if you think it is a useful addition.

[skyace65]

Thanks! And congrats on your first merged PR!