Explain optional buffer.uri

@lexaknyazev I was looking at [Buffer's reference](https://www.khronos.org/registry/glTF/specs/2.0/glTF-2.0.html#reference-buffer), and could not find explanation regarding why `uri` field might be optional. Maybe a note should be added at this point in the reference?

_Originally posted by @Adnn in https://github.com/KhronosGroup/glTF/issues/828#issuecomment-1058934103_