Update guidance on relevant vs. current settings objects

[domenic]

Per previous discussions with @bzbarsky, he believes that the relevant settings object of this is more generally what should be used as the default, instead of the current settings object. This mismatches ECMAScript (which is why I originally preferred current settings object as the default), but in w3c/css-houdini-drafts#251 (comment) onward I remember him convincing me that relevant was a better default for some reason, and that's what we've been moving toward in specs since then. Also, there's the precedent set by whatwg/webidl#135, which is pretty prevalent.

We should also note how in constructors you should use current, since there is no object to get the relevant settings object of.