Update documentation copy + link

[cwervo]

Changes Proposed:

Doesn't affect much, but when you do use the WebVR 2.0 link in this explainer this changes it to the right path: xr instead of vr.

Also makes some edits where wording was unclear/verbose. Not comprehensive, just some notes I had while reading through it the first time.

Fixes:

N/A

Checklist:

• [x] I've read CONTRIBUTING.md and signed the CLA (required)
• [x] This PR does not contain built changes in dist/\* (required)
• [x] This PR only contains one commit (required)

[jsantell]

Thanks for this, @AndresCuervo!