CanvasModulate documentation does not describe what happens if there are multiple instances but only 1 has visible set to true

**Your Godot version:**
v4.2.1.stable.official [b09f793f5]

**Issue description:**
The CanvasModulate docs state: "Only one can be used to tint a canvas, but CanvasLayers can be used to render things independently." This limitation is reflected in the engine; if you add more than one in the same CanvasLayer, you get a warning. The warning says which one is active is undefined if there's more than 1. However, if you make all but 1 CanvasModulate node invisible, the warning goes away, and the behaviour is predictable (i.e., the expected colour is applied).

Therefore, the warning and the docs are incomplete. They need to be updated to describe what happens when there are multiple nodes, specifically, what happens when all but 1 are visible=false.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_canvasmodulate.html