Document admonitions in GDScript documentation comments

[Calinou]

• Should be merged after Automatically add a line break before each admonition in GDScript doc comments godot#123353.

If we decide not to go forward with the above PR, we should add this to the docs:

.. warning::

    Admonitions do not display on separate paragraphs automatically. You must
    add **two** explicit line breaks with ``[br][br]`` before the admonition to
    ensure it is displayed correctly.

    ::

        ## Example description.
        ## [br][br][note]This is a note.[/note]
        ## [br][br][warning]This is a warning.[/warning]
        @export var example = ""

[Cykyrios]

LGTM. If godotengine/godot#123357 gets merged, the example should be updated to add empty comment lines between each paragraph instead.

[Calinou]

LGTM. If godotengine/godot#123357 gets merged, the example should be updated to add empty comment lines between each paragraph instead.

That PR is now merged, but if godotengine/godot#123353 is also merged, the docs should be valid in their current state already.