Document how to run logic after returning a response

[mrkurt]

Like when caching:

let resp = fetch("...")
inSweetNonBlockingFashion(function(){ cache.put(req, resp) });
return resp

[jeromegn]

Do we need to document this or is it fine since the recent "runtime" changes? (more intuitive)

[mrkurt]

I'm not clear on if a promise without await works well for this, or people would be better off with setTimeout. They both seem to work now, though, so probably less docs needed.