v3(docs): document last operation for service key bindings

[#174252663](https://www.pivotaltracker.com/story/show/174252663)

Author: pivotal-marcela-campo

docs/v3/source/includes/api_resources/_service_credential_bindings.erb

@@ -50,7 +50,12 @@
   "updated_at": "2016-06-08T16:41:26Z",
   "name": "some-name",
   "type": "key",
-  "last_operation": null,
+  "last_operation": {
+    "type": "create",
+    "state": "succeeded",
+    "created_at": "2015-11-13T17:02:56Z",
+    "updated_at": "2016-06-08T16:41:26Z"
+  },
   "relationships": {
     "service_instance": {
       "data": {

docs/v3/source/includes/experimental_resources/service_credential_bindings/_object.md.erb

@@ -12,7 +12,7 @@ Name | Type | Description
 **guid** | _uuid_ | Unique identifier for the service credential binding
 **name** | _string_ | Name of the binding. `null` when it's not defined.
 **type** | _string_ | Either `app` or `key`
-**last_operation** (*experimental*)| _[last operation object](#the-last-operation-object-for-service-credential-binding)_ | The last operation of this binding; `null` when type is `key`
+**last_operation** (*experimental*)| _[last operation object](#the-last-operation-object-for-service-credential-binding)_ | The last operation of this binding
 **created_at** | _[timestamp](#timestamps)_ | The time with zone when the object was created
 **updated_at** | _[timestamp](#timestamps)_ | The time with zone when the object was last updated
 **relationships.service_instance** | [_to-one relationship_](#to-one-relationships) | The service instance that this binding is originated from