Note Elixir 1.19+ requirement for tuple :keys forms

Author: studzien

lib/phoenix/pubsub.ex

@@ -184,8 +184,8 @@ defmodule Phoenix.PubSub do
       processes; it is exposed for completeness. The two duplicate
       variants differ in partitioning: `{:duplicate, :pid}` groups
       entries by subscriber pid, while `{:duplicate, :key}` groups them
-      by topic. See `Registry.start_link/1` for the `:pid` vs `:key`
-      trade-offs.
+      by topic. The tuple forms require Elixir v1.19 or later. See
+      `Registry.start_link/1` for the `:pid` vs `:key` trade-offs.
 
   """
   @spec child_spec(keyword) :: Supervisor.child_spec()