The documentation should contain a line about abstract native classes

**Your Godot version:** v4.6.3.stable.official [7d41c59c4]

**Issue description:**

It stems from this issue https://github.com/godotengine/godot/issues/119865

The documentation does not mention abstract native classes, and that they're different from "normal" abstract classes.

For instance, you won't be able to instantiate a class extending an abstract native class, while you could with a "normal" abstract class, but you can still attach a script extending it.

**URL to the documentation page (if already existing):**

https://docs.godotengine.org/en/stable/tutorials/scripting/gdscript/gdscript_basics.html

Open the linked gdscript_basics.html page and find the existing discussion of abstract classes. Add a concise note distinguishing abstract native classes from normal abstract classes, including the instantiation and script-attachment behavior described in the issue. Confirm the updated documentation renders correctly.

Written by the indexing model from the issue text.