Clarify TRANSFORM_FEEDBACK_BUFFER_BINDING state

[lexaknyazev]

OpenGL ES 3.2, BindTransformFeedback:

The TRANSFORM_FEEDBACK_BUFFER_BINDING generic buffer binding point has now been moved to context state, rather than per-object state, and should be unaffected by this command. It is possible that some drivers do not yet implement this change.

Based on the implementations, it looks like the Differences Between WebGL and OpenGL ES 3.0 section must contain a similar statement since OpenGL ES 3.0 defines the generic binding point per-object.

Moreover, the existing Preventing undefined behavior with Transform Feedback section should clearly state which bindings (generic, indexed, or both) it refers to.