[DX] Document the environment variables supported by the console script

[stof]

The console script provided in the SE allows to choose the environment and the debug mode through environment variables rather than using options. This should be documented as it is very useful when automating things or when running some commands during the composer install (which happens in the default setup)

See symfony/symfony#11704 for one of the discussions olved by saying "these environment variables allow to do it"

[javiereguiluz]

For reference purposes, these are the specific code lines that show how does this work: https://github.com/symfony/symfony-standard/blob/master/app/console#L18-19

[wouterj]

This should be documented in sections where we talk about such commands (e.g. book/installation)

[javiereguiluz]

I've prepared the following simple explanation of these two variables, but I don't really know where to put it. @xabbuh @wouterj @weaverryan I need your help deciding the best place (and tweaking the proposed doc):

In addition to ``--env`` and ``--debug`` options, Symfony commands behavior can
also be controlled with environment variables. The Symfony console application
checks the existence and value of these environment variables before executing
any command:

* ``SYMFONY_ENV``, sets the environment in which the command is executed, 
  which is usually ``dev`` or ``prod``.
* ``SYMFONY_DEBUG`, controls whether the command should enable the debug features
  of Symfony, which is usually ``true`` for the ``dev`` environment and ``false``
  for the ``prod`` environment.

[xabbuh]

What about adding it to http://symfony.com/doc/current/cookbook/configuration/environments.html (specifically under "Executing an Application in different Environments")?

[javiereguiluz]

@xabbuh thanks for your suggestion. I've added the new docs in #4665.