ViewportTextureNode: Improve docs and clean up.

[Mugen87]

Related issue: #31591

Description

Adding some more docs to ViewportTextureNode.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.74 79.06	338.74 79.06	+0 B +0 B
WebGPU	568.16 157.13	568.2 157.14	+39 B +7 B
WebGPU Nodes	566.77 156.89	566.81 156.89	+39 B +7 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.48 113.82	470.48 113.82	+0 B +0 B
WebGPU	641.87 173.61	641.9 173.61	+38 B +6 B
WebGPU Nodes	596.51 162.91	596.55 162.92	+38 B +11 B