Add documentation for the env processor

Author: Orkin

configuration/environment_variables.rst

@@ -510,6 +510,9 @@ Symfony provides the following env var processors:
 
         The ``trim`` processor was introduced in Symfony 4.3.
 
+``env(explode:FOO,BAR)``
+    Explode the content of ``FOO,BAR`` env var, retrieving a php array.
+
 ``env(key:FOO:BAR)``
     Retrieves the value associated with the key ``FOO`` from the array whose
     contents are stored in the ``BAR`` env var: