Clarify rules around IDL method arguments

[dontcallmedom]

As discussed in w3c/reffy#336 (comment), bikeshed and respec aren't aligned as data producers nor as data consumers when it comes to including arguments in definitions. Some of the constraints around the topic are linked with overloaded methods, although the fact that they are now mandated to be defined in a single IDL fragment may help on the producer side.

[dontcallmedom]

Relevant bikeshed issue speced/bikeshed#1733

[marcoscaceres]

WebIDL says we should support the form foo(bar, bas) etc., so I'd be inclined to support that (and I agree it's better than what we have now in ReSpec)... just haven't gotten around to fully coding it up: https://github.com/w3c/respec/issues/3571