Update preload_app! comment in dsl.rb

The default changed in Puma 5 with puma#2143.

lib/puma/dsl.rb

@@ -561,7 +561,7 @@ def directory(dir)
     end
 
     # Preload the application before starting the workers; this conflicts with
-    # phased restart feature. This is off by default.
+    # phased restart feature. On by default if your app uses more than 1 worker.
     #
     # @note Cluster mode only.
     # @example