PointsNodeMaterial: Clarify docs for sizeNode.

[Mugen87]

Fixed #31527.

Description

The PR improves the JSDoc for sizeNode by highlighting that the property has no effect when using with THREE.Points and a WebGPU backend.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.39 78.91	338.39 78.91	+0 B +0 B
WebGPU	566.16 156.5	566.16 156.5	+0 B +0 B
WebGPU Nodes	564.77 156.26	564.77 156.26	+0 B +0 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	469.82 113.62	469.82 113.62	+0 B +0 B
WebGPU	637.38 172.47	637.38 172.47	+0 B +0 B
WebGPU Nodes	592.02 161.71	592.02 161.71	+0 B +0 B