Documentation: distinguish Abstract/Virtual base classes that can't be instantiated directly

**Your Godot version:**
3.3.2

**Issue description:**
`Script.new()` gives `Class type: 'Script' is not instantiable` error. I didn't expect that because this isn't specified in the docs. For example `BaseButton` has such note in the class description:
```
BaseButton is the abstract base class for buttons, so it shouldn't be used directly (it doesn't display anything).
Other types of buttons inherit from it.
```

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_script.html?highlight=script