add response example and 'extra' info of create user

1.add response example of create user.
2.add 'email' and 'description' info in request example to keep consistent with
response example.
3.add usage introduction of 'extra' column.

Change-Id: Idb29a73b127111c9651491f1db5b0e91de9e1063

api-ref/source/v3/parameters.yaml

@@ -849,6 +849,18 @@ explicit_unscoped_string:
   in: body
   required: false
   type: string
+extra_request_body:
+  description: |
+    The extra attributes of a resource.
+    The actual name ``extra`` is not the key name in the request body,
+    but rather a collection of any attributes that a resource may contain
+    that are not part of the resource's default attributes.
+    Generally these are custom fields that are added to a resource in keystone
+    by operators for their own specific uses,
+    such as ``email`` and ``description`` for users.
+  in: body
+  required: false
+  type: string
 group:
   description: |
     A ``group`` object, containing:

api-ref/source/v3/samples/admin/user-create-request.json

@@ -4,6 +4,8 @@
         "domain_id": "1789d1",
         "enabled": true,
         "name": "James Doe",
-        "password": "secretsecret"
+        "password": "secretsecret",
+        "description": "James Doe user",
+        "email": "jdoe@example.com"
     }
 }

api-ref/source/v3/users.inc

@@ -103,6 +103,7 @@ Parameters
    - enabled: enabled_user_request_body
    - name: user_name_create_request_body
    - password: password_request_body
+   - extra: extra_request_body
 
 Example
 ~~~~~~~
@@ -141,6 +142,12 @@ Status Codes
    - 403
    - 409
 
+Example
+~~~~~~~
+
+.. literalinclude:: ./samples/admin/user-create-response.json
+   :language: javascript
+
 
 Show user details
 =================