minor symfony#10470 [server:run] [symfony#10461] Add docs about custom environment (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
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[server:run] [symfony#10461] Add docs about custom environment

| Q             | A
| ------------- | ---
| Fixed tickets | symfony#10461, symfony/symfony-docs#3688
| License       | MIT

It's better to document this specific usecase in the command help.

Commits
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6871a61 [symfony#10461] Add docs about custom environment

Author: fabpot

src/Symfony/Bundle/FrameworkBundle/Command/ServerRunCommand.php

@@ -66,6 +66,9 @@ protected function configure()
 router script using <info>--router</info> option:
 
   <info>%command.full_name% --router=app/config/router.php</info>
+  
+Specifing a router script is required when the used environment is not "dev" or
+"prod".
 
 See also: http://www.php.net/manual/en/features.commandline.webserver.php
 EOF