feature #4665 Documented the console environment variables (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
----------

Documented the console environment variables

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4162

Commits
-------

df2a0f2 Fixes and tweaks
dd8157f More tweaks and grammar fixes
2fb082d Rewordings and transformed a list into a definition list
a1d88d8 More grammar fixes
8ead4a4 Minor grammar fix
6fc8f4c Fixed some errors in the explanation
7c4cd17 Documented the console environment variables

Author: weaverryan

cookbook/configuration/environments.rst

@@ -212,6 +212,39 @@ environment by using this code and changing the environment string.
     mode. You'll need to enable that in your front controller by calling
     :method:`Symfony\\Component\\Debug\\Debug::enable`.
 
+Selecting the Environment for Console Commands
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+By default, Symfony commands are executed in the ``dev`` environment and with the
+debug mode enabled. Use the ``--env`` and ``--no-debug`` options to modify this
+behavior:
+
+.. code-block:: bash
+
+    # 'dev' environment and debug enabled
+    $ php app/console command_name
+
+    # 'prod' environment (debug is always disabled for 'prod')
+    $ php app/console command_name --env=prod
+
+    # 'test' environment and debug disabled
+    $ php app/console command_name --env=test --no-debug
+
+In addition to the ``--env`` and ``--debug`` options, the behavior of Symfony
+commands 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 execution environment of the command to the value of this variable
+    (``dev``, ``prod``, ``test``, etc.);
+``SYMFONY_DEBUG``
+    If ``0``, debug mode is disabled. Otherwise, debug mode is enabled.
+
+These environment variables are very useful for production servers because they
+allow you to ensure that commands always run in the ``prod`` environment without
+having to add any command option.
+
 .. index::
    single: Environments; Creating a new environment