Clarify "cannot be applied at the media layer"

[stefhak]

As reported in https://lists.w3.org/Archives/Public/public-webrtc/2016Sep/0071.html

"What does “cannot be applied at the media layer” mean"

[stefhak]

@fluffy the sentence reads:
For all other errors, for example if description cannot be applied at the media layer, reject p with a newly created OperationError., i.e. "cannot be applied at the media layer" is just an example. Would you be OK with removing that part (i.e. , for example if description cannot be applied at the media layer)?

I'm also labeling this as "Editorial" since anyone not understanding this could just ignore it.