Docs: Improve ShaderMaterial.fog.

[Mugen87]

Fixed #32206.

Description

Slightly enhances the docs for ShaderMaterial.fog to clarify the intended usage.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	356.24 86.47	356.24 86.47	+0 B +0 B
WebGPU	610.57 171.28	610.57 171.28	+0 B +0 B
WebGPU Nodes	609.18 171.02	609.18 171.02	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	488.03 121.24	488.03 121.24	+0 B +0 B
WebGPU	679.51 186.54	679.51 186.54	+0 B +0 B
WebGPU Nodes	621.23 169.71	621.23 169.71	+0 B +0 B

[Mugen87]

Had to update some E2E screenshots because of 74cd7fd.

[Mugen87]

I've tried three times to update the screenshot for webgl_postprocessing_glitch. I've actually used the "expected" screenshots from the CI but with no luck. Adding the demo to the exception list.