CollisionShape2D: Document outline-only debug color

**Your Godot version:**

v4.7.2.stable.flathub [ed1daf0bf]

**Issue description:**

I was very surprised (and happy) to find out that the primitive collision shapes in 2D can have a debug color that has full alpha, to convey an outline-only effect:

The picture shows the same shapes, one with Debug Color set to `#ff001800` on the left (zero alpha) and on the right is set to `#ff00186b` (42% alpha). My colleague @wjt found in the source code that in fact the outline is always solid: https://github.com/endlessm/threadbare/pull/2776#pullrequestreview-5130336230

I think it would be nice to document this, because when stacking multiple shapes (a physics collider, a hitbox, an interaction shape...) they start to become unreadable. Either in the Physics Collision shapes (2D) guide, or in the class itself.

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

https://docs.godotengine.org/en/stable/tutorials/physics/collision_shapes_2d.html

Start with the Physics Collision shapes (2D) guide at the linked documentation URL and review how Debug Color is currently described; the issue also suggests documenting this in the relevant class reference. Add a note that a fully transparent debug color can still show a solid outline, including the outline-only use case, and verify the wording renders correctly in the documentation.

Written by the indexing model from the issue text.