addition to ActionCable documentation re: application servers with multiple workers

Hi guys,

I would try adding this note to the ActionCable documentation, but I don’t think I could find the perfect words, and also wouldn’t be certain how to generalize the issue beyond my specific situation (i.e. I was using Puma, but I’m not sure how other application servers would be affected by this).

I spent a few hours banging my head on the wall with ActionCable because I was using async in development mode with Puma and had multiple workers. Depending on which worker each javascript client was connecting to, they were effectively in siloed subscriptions.