Make mousewheel/trackpad scrolling in Quick Docs less painful

When cursor is over Quick Docs details area, never let mousewheel/trackpad scroll the host editor.  (If the docs scroller is already maxed out, do nothing at all).

In theory we could do something fancier... e.g. remember when the last host editor scroll event event occurred, as a heuristic to detect whether you're trying to scroll the docs specifically vs. just passing over the docs while scrolling the host editor.  But I think what's implemented here is "good enough" for now.