Document the fact that Puma 5.0+ now pulls from WEB_CONCURRENCY directly

Since we no longer have a block about worker settings in the file.

Author: dhh

railties/lib/rails/generators/rails/app/templates/config/puma.rb.tt

@@ -5,6 +5,10 @@
 # Puma starts a configurable number of processes (workers) and each process
 # serves each request in a thread from an internal thread pool.
 #
+# You can control the number of workers using ENV["WEB_CONCURRENCY"]. You
+# should only set this value when you want to run 2 or more workers. The
+# default is already 1.
+#
 # The ideal number of threads per worker depends both on how much time the
 # application spends waiting for IO operations and on how much you wish to
 # prioritize throughput over latency.