Updated configuring service manager documentation

Invokables are used in zend-expressive a lot and these were missing in
the docs

Author: michalbundyra

docs/book/configuring-the-service-manager.md

@@ -4,21 +4,24 @@ The Service Manager component can be configured by passing an associative array
 constructor. The following keys are:
 
 - `services`: associative array that maps a key to a service instance.
+- `invokables`: an associative array that map a key to a constructor-less service;
+  i.e., for services that do not require arguments to the constructor. The key and
+  service name usually are the same; if they are not, the key is treated as an alias.
 - `factories`: associative array that map a key to a factory name, or any callable.
 - `abstract_factories`: a list of abstract factories classes. An abstract
   factory is a factory that can potentially create any object, based on some
   criterias.
 - `delegators`: an associative array that maps service keys to lists of delegator factory keys, see the [delegators documentation](delegators.md) for more details.
 - `aliases`: associative array that map a key to a service key (or another alias).
 - `initializers`: a list of callable or initializers that are run whenever a service has been created.
-- `shared`: associative array that map a service name to a boolean, in order to
-  indicate the service manager if it should cache or not a service created
-  through the `get` method, independant of the `shared_by_default` setting.
 - `lazy_services`: configuration for the lazy service proxy manager, and a class
   map of service:class pairs that will act as lazy services; see the
   [lazy services documentation](lazy-services.md) for more details.
+- `shared`: associative array that map a service name to a boolean, in order to
+  indicate the service manager if it should cache or not a service created
+  through the `get` method, independent of the `shared_by_default` setting.
 - `shared_by_default`: boolean that indicates whether services created through
-  the `get` method should be cached. This is true by default.
+  the `get` method should be cached. This is `true` by default.
 
 Here is an example of how you could configure a service manager:
 
@@ -27,11 +30,15 @@ use Zend\ServiceManager\ServiceManager;
 
 $serviceManager = new ServiceManager([
     'services'           => [],
+    'invokables'         => [],
     'factories'          => [],
     'abstract_factories' => [],
     'delegators'         => [],
+    'aliases'            => [],
+    'initializers'       => [],
+    'lazy_services'      => [],
     'shared'             => [],
-    'shared_by_default'  => true
+    'shared_by_default'  => true,
 ]);
 ```
 
@@ -50,12 +57,13 @@ use stdClass;
 
 $serviceManager = new ServiceManager([
     'factories' => [
-        stdClass::class => InvokableFactory::class
-    ]
+        MyObject::class => MyObjectFactory::class,
+    ],
 ]);
 ```
 
-> This mechanism replaces the `invokables` key that was used in Zend Framework 2.
+> For invokable classes we can use `Zend\ServiceManager\Factory\InvokableFactory`
+> but for performance reasons using `invokables` configuration is recommended.
 
 As said before, a factory can also be a callable, to create more complex objects:
 
@@ -95,6 +103,16 @@ class MyObjectFactory implements FactoryInterface
     }
 }
 
+// or without implementing the interface:
+class MyObjectFactory
+{
+    public function __invoke(ContainerInterface $container, $requestedName)
+    {
+        $dependency = $container->get(Dependency::class);
+        return new MyObject($dependency);
+    }
+}
+
 // When creating the service manager:
 $serviceManager = new ServiceManager([
     'factories' => [
@@ -130,6 +148,16 @@ class MyObjectFactory implements FactoryInterface
     }
 }
 
+// or without implementing the interface:
+class MyObjectFactory
+{
+    public function __invoke(ContainerInterface $container, $requestedName)
+    {
+        $dependency = $container(Dependency::class);
+        return new $requestedName($dependency);
+    }
+}
+
 // When creating the service manager:
 $serviceManager = new ServiceManager([
     'factories' => [