Hover behavior is inconsistent between diagnostics and docs

There are two kinds of hover popups that are shown: (a) doc-on-hover for known symbols, and (b) error highlights. The two don't act the same. Error highlight popups follow the mouse pointer, but doc popups are positioned relative to the span being hovered on. These should act the same way. I prefer the doc behavior, where the popup is placed relative to the span, not the mouse pointer.

Not a big deal, but the inconsistency is not great for user experience.

@nixorn You implemented these, so if you've got time, this might be easier for you to fix than me.