minor #14040 Add note about php-fpm being installed locally (kbond)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead.

Discussion
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Add note about php-fpm being installed locally

I was a little confused about how to get the server to use FPM instead of the PHP's built-in webserver.

Commits
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9d0312f add note about php-fpm being installed locally

Author: javiereguiluz

setup/symfony_server.rst

@@ -60,6 +60,10 @@ run the Symfony server in the background:
 Enabling PHP-FPM
 ----------------
 
+.. note::
+
+    PHP-FPM must be installed locally for the Symfony server to utilize.
+
 When the server starts it will check for common patterns like ``web/app.php``,
 ``web/app_dev.php`` or ``public/index.php``. If a file like this is found the
 server will automatically start with PHP-FPM enabled. Otherwise the server will