Mention why we use multiple AdaptationSet for audio

src/invidious/routes/api/manifest.cr

@@ -64,6 +64,10 @@ module Invidious::Routes::API::Manifest
               # OTF streams aren't supported yet (See https://github.com/TeamNewPipe/NewPipe/issues/2415)
               next if !(fmt.has_key?("indexRange") && fmt.has_key?("initRange"))
 
+              # Different representations of the same audio should be groupped into one AdaptationSet.
+              # However, most players don't support auto quality switching, so we have to trick them
+              # into providing a quality selector.
+              # See https://github.com/iv-org/invidious/issues/3074 for more details.
               xml.element("AdaptationSet", id: i, mimeType: mime_type, startWithSAP: 1, subsegmentAlignment: true, label: fmt["bitrate"].to_s + "k") do
                 codecs = fmt["mimeType"].as_s.split("codecs=")[1].strip('"')
                 bandwidth = fmt["bitrate"].as_i