Explicitly document that body attr will be ignored when method is GET or HEAD

[jonvuri]

Since iron-ajax / -request simply proxy down to XHR, even if a body attribute is set, it will simply be ignored, and not sent with the request. This isn't indicated very clearly in the docs for the body attribute.

[jonvuri]

Ref in spec https://xhr.spec.whatwg.org/#the-send()-method

[rictic]

Hm, my intuition is that if we documented everything that iron-ajax did, the documentation would be as long as the spec itself! OTOH this is a fairly reasonable thing to document. I'd accept a PR documenting this.

On a related note, it looks like the fetch API won't fix this either, though it will at least give a useful error message rather than silently setting body to null. See step 31 here: https://fetch.spec.whatwg.org/#request-class