minor #10983 Added a short section about "Setting Environment Variables for Processes" (javiereguiluz)

This PR was squashed before being merged into the 4.2 branch (closes #10983).

Discussion
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Added a short section about "Setting Environment Variables for Processes"

Fixes #10967.

Commits
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91697b8 Added a short section about \"Setting Environment Variables for Processes\"

Author: javiereguiluz

components/process.rst

@@ -128,6 +128,27 @@ environment variables using the second argument of the ``run()``,
     // On both Unix-like and Windows
     $process->run(null, ['MESSAGE' => 'Something to output']);
 
+Setting Environment Variables for Processes
+-------------------------------------------
+
+The constructor of the :class:`Symfony\\Component\\Process\\Process` class and
+all of its methods related to executing processes (``run()``, ``mustRun()``,
+``start()``, etc.) allow passing an array of environment variables to set while
+running the process::
+
+    $process = new Process(['...'], null, ['ENV_VAR_NAME' => 'value']);
+    $process = Process::fromShellCommandline('...', null, ['ENV_VAR_NAME' => 'value']);
+    $process->run(null, ['ENV_VAR_NAME' => 'value']);
+
+In addition to the env vars passed explicitly, processes inherit all the env
+vars defined in your system. You can prevent this by setting to ``false`` the
+env vars you want to remove::
+
+    $process = new Process(['...'], null, [
+        'APP_ENV' => false,
+        'SYMFONY_DOTENV_VARS' => false,
+    ]);
+
 Getting real-time Process Output
 --------------------------------