Add paragraph about buffer object binding in WebGL 2

There's more buffer binding points and functions that set buffer binding
points in WebGL 2. Restricting index buffers not being bound to other
binding points avoids possible security issues with generating indices
from transform feedback.
Fix absolute links and add anchors for consistency in WebGL 2 spec
Merge pull request #630 from NVIDIA/webgl2-buffer-object-binding

Add paragraph about buffer object binding in WebGL 2