Follow-ups to #1463

• Expand https://gpuweb.github.io/gpuweb/#depth-formats with extension formats
• Clarify wording in that section that says "depth" when it really means "depth and/or stencil"?
• Consider explicitly documenting that T2T copies with matching format is always valid (all formats, not just D/S) Contradictions in the validation rules of copyTextureToTexture() with depth stencil formats #1464