Fix concave `CollisionPolygon2D` debug drawing and docs

[rburing]

The debug drawing for CollisionPolygon2D showed the convex decomposition unconditionally, which was meaningless in Segments build mode. The dead code would show a filled polygon, which would also be misleading as you can just as well put a character on the inside (because the shape is purely built of segments), so this PR draws nothing on the inside when the build mode is Segments. (If people do prefer a filled polygon, we could do that instead.)

https://github.com/godotengine/godot/assets/229837/07f2f1f8-a7f7-4d2d-b03a-fa677b048617

The documentation had some copy/paste error from the 3D analogue, and it was mentioning only the solid option; now hollow is mentioned as well.

• Closes https://github.com/godotengine/godot/issues/10110

[rburing]

The empty inside would be consistent with ConcavePolygonShape2D.

[akien-mga]

Thanks!