Description for everyone else

This PR adds a description for the EdX Video ID field, which everyone will see, to the Building and Running an edX Course guide.

@nasthagiri, @frrrances, @WatsonEmily , @lou-wang, @srpearce 
@mhoeber, @catong FYI
Just to make this more consistent with the other items, I'd change slightly: "An optional field used only by course teams..."