Update the WebIDL guidance for explainers

[jyasskin]

The template says not to include WebIDL at all. The guide currently says to "use code examples rather than prose", and w3ctag/tag.w3.org#66 adds a recommendation for screenshots and diagrams, and removes the "rather than prose".

@cynthia mentions that we should probably address other kinds of interface definitions besides just WebIDL.

As a design gets fleshed out, it's likely that the proponents will have some WebIDL before they're ready to write a full spec: we should say what to do with that IDL. Is it ok to put it lower down in the explainer, or do we want them to start a spec as soon as they have any IDL?

[jyasskin]

Whoops, #15 also covers part of this.