minor #3641 Added some examples to the "services as parameters" secti…

…on (javiereguiluz)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3641).

Discussion
----------

Added some examples to the "services as parameters" section

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | no

Commits
-------

745f3a6 Removed the file path comments because this doc is for the component and it should not make any reference to Symfony
6abb310 Added some examples to the "services as parameters" section

components/dependency_injection/parameters.rst

@@ -343,6 +343,15 @@ Start the string with  ``@`` or ``@?`` to reference a service in YAML.
 * ``@?mailer`` references the ``mailer`` service. If the service does not
   exist, it will be ignored;
 
+.. code-block:: yaml
+
+    parameters:
+        # if 'my_mailer' service isn't defined, an exception will be raised
+        foo: @my_mailer
+
+        # if 'my_logger' service isn't defined, 'bar' will be null
+        bar: @?my_logger
+
 .. tip::
 
     Use ``@@`` to escape the ``@`` symbol in YAML. ``@@mailer`` will be
@@ -358,6 +367,16 @@ is thrown. Valid values for ``on-invalid`` are ``null`` (uses ``null`` in place
 of the missing service) or ``ignored`` (very similar, except if used on a
 method call, the method call is removed).
 
+.. code-block:: xml
+
+    <parameters>
+        <!-- if 'my_mailer' service isn't defined, an exception will be raised -->
+        <parameter key="foo" type="service" id="my_mailer" />
+
+        <!-- if 'my_logger' service isn't defined, 'bar' will be null -->
+        <parameter key="bar" type="service" id="my_logger" on-invalid="null" />
+    </parameters>
+
 PHP
 ~~~
 
@@ -366,3 +385,15 @@ In PHP, you can use the
 a service. The invalid behavior is configured using the second constructor
 argument and constants from
 :class:`Symfony\\Component\\DependencyInjection\\ContainerInterface`.
+
+.. code-block:: php
+
+    use Symfony\Component\DependencyInjection\Reference;
+
+    // if 'my_mailer' service isn't defined, an exception will be raised
+    $container->setParameter('foo', new Reference('my_mailer'));
+
+    // if 'my_logger' service isn't defined, 'bar' will be null
+    $container->setParameter('bar', new Reference('my_logger',
+        ContainerInterface::NULL_ON_INVALID_REFERENCE
+    ));