Add non-normative description to request() method definition

[inexorabletash]

Due to the complexity around handling the dict arguments for request(), the algorithm is even harder to read than in most specs. To be clear, I think the text is correct, it's just difficult to figure out why it's written that way and what it's doing - I had to dredge up my memories of seeing a thread somewhere.

Adding some non-normative text there would help - either notes inline in the algorithm, or "domintro" style blocks that are popping up in more specs (e.g. the green block in https://dom.spec.whatwg.org/#interface-customevent as an example), or both.