Document workaround for clamped additive lighting with Light2D nodes

**Your Godot version:** 3.2.2

**Issue description:**

There's a workaround that can be used to achieve clamped additive lighting with Light2D nodes. However, it needs to be documented.

See this proposal and its first comment: https://github.com/godotengine/godot-proposals/issues/891

**URL to the documentation page (if already existing):** https://docs.godotengine.org/en/latest/tutorials/2d/2d_lights_and_shadows.html in addition to the Light2D class reference.