Docs: Update WebXRManager methods

[harryhjsh]

Description

Adds missing WebXRManager methods to old docs. Not sure what the status of the new docs is but thought this was worth doing? Also has some smaller JSDoc fixes/clarifications.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.96 79.12	338.96 79.12	+0 B +0 B
WebGPU	575.35 158.76	575.35 158.76	+0 B +0 B
WebGPU Nodes	573.96 158.51	573.96 158.51	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.7 113.89	470.7 113.89	+0 B +0 B
WebGPU	646.42 174.83	646.42 174.83	+0 B +0 B
WebGPU Nodes	600.6 164.01	600.6 164.01	+0 B +0 B