Unfortunately it's not a patch on Dash. I used to use it, but found too many cases where it was missing docsets and glitching on existing ones. It didn't handle different screen resolutions well. The Fedora package for it is often broken (I just installed it to see if things have improved & all I get is an empty window with an odd partial-width window title bar).

There's a gap in the market for a good cross-platform docs browser, preferably supporting user-added notes.