Clarify that the predefined report types all use the `default` group

[dcreager]

For all of the current predefined report types (deprecation, intervention, and crash), there's no way to tell the browser which endpoint group to use for reports that it creates — it will always use the default group. We should clarify the text to say that, but I'm not sure the best way to do it. For other report types defined elsewhere (NEL, CSP), there's an algorithm somewhere that includes a step "Queue the report for delivery using the Reporting API", and part of that step is an explicit description of which endpoint group to use (regardless of whether it's from some configuration header or hard-coded).

It's probably overkill to add a bunch of algorithms just for this, so maybe just a note?

/cc @paulmeyer90