[DependencyInjecion] Mention that service definitions replace previous ones

Author: javiereguiluz

service_container.rst

@@ -178,6 +178,9 @@ each time you ask for it.
                         - '../src/Entity/'
                         - '../src/Kernel.php'
 
+                # order is important in this file because service definitions
+                # always *replace* previous ones; add your own service configuration below
+
                 # ...
 
         .. code-block:: xml
@@ -197,6 +200,9 @@ each time you ask for it.
                     <!-- this creates a service per class whose id is the fully-qualified class name -->
                     <prototype namespace="App\" resource="../src/" exclude="../src/{DependencyInjection,Entity,Kernel.php}"/>
 
+                    <!-- order is important in this file because service definitions
+                         always *replace* previous ones; add your own service configuration below -->
+
                     <!-- ... -->
 
                 </services>
@@ -219,6 +225,9 @@ each time you ask for it.
                 // this creates a service per class whose id is the fully-qualified class name
                 $services->load('App\\', '../src/')
                     ->exclude('../src/{DependencyInjection,Entity,Kernel.php}');
+
+                // order is important in this file because service definitions
+                // always *replace* previous ones; add your own service configuration below
             };
 
     .. tip::