Clarify GDExtension GDRegister types in C++ Tutorial

[ColinSORourke]

Adds a clarifying note about the different types of GDRegister in the GDExtension C++ Tutorial.

Based on Issue: godotengine/godot#107195

[Ivorforce]

Note that this is also explained elsewhere:
https://docs.godotengine.org/en/latest/engine_details/architecture/object_class.html#registering-object-classes

Which is a page linked from the godot-cpp docs:
https://docs.godotengine.org/en/latest/tutorials/scripting/cpp/core_types.html

I'm guessing one reason for the confusion could be that the info is sort of new and hasn't been cherry-picked to stable yet? (#11447 and #11438)

[ColinSORourke]

@Ivorforce Great catch, thanks for pointing those out. There's still a case to be made for this brief warning, or perhaps just adding one sentence: "More info on Object Registration methods can be found [link]"

It definitely appears you're right, those PRs hadn't even been created when Issue godotengine/godot#107195 was posted, but with that I'm certainly satisfied closing the original issue, and adjusting or closing this PR.

[ColinSORourke]

Thanks for the feedback @AThousandShips @Ivorforce - just changed to Ivor's suggestion of just using GDRegister_Class, with the simpler note. Let me know if any formatting or wording still needs to be adjusted.

[Ivorforce]

Looks good to me!

[paddy-exe]

Looks good 👍🏻

[mhilbrunner]

Thank you, and thanks for the reviews everyone!