Clarify PID file configuration in development

Author: rafaelfranca

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

@@ -33,5 +33,6 @@ port ENV.fetch("PORT", 3000)
 # Allow puma to be restarted by `bin/rails restart` command.
 plugin :tmp_restart
 
-# Only use a pidfile when requested
+# Specify the PID file. Defaults to tmp/pids/server.pid in development.
+# In other environments, only set the PID file if requested.
 pidfile ENV["PIDFILE"] if ENV["PIDFILE"]