docs(worker): comment purpose of UI refresh task

The purpose of the manual UI refresh task isn't immediately clear. It
was explained in the commit message of the commit that introduced it.

src/spotify_worker.rs

@@ -186,6 +186,7 @@ impl Worker {
                     },
                     _ => {}
                 },
+                // Update animated parts of the UI (e.g. statusbar during playback).
                 _ = ui_refresh.tick() => {
                     if self.active {
                         self.events.trigger();