Update domains docs to include link to router groups when necessary

* mark router groups field on request as experimental as per
[story](https://www.pivotaltracker.com/story/show/169276053)

[finishes #172547117]

Authored-by: Mona Mohebbi <mmohebbi@pivotal.io>

Author: monamohebbi

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

@@ -82,6 +82,9 @@
         },
         "route_reservations": {
           "href": "https://api.example.org/v3/domains/3a5d3d89-3f89-4f05-8188-8a2b298c79d5/route_reservations"
+        },
+        "router_group": {
+          "href": "https://api.example.org/routing/v1/router_groups/5806148f-cce6-4d86-7fbd-aa269e3f6f3f"
         }
       }
     },

docs/v3/source/includes/resources/domains/_create.md.erb

@@ -40,10 +40,10 @@ Name     | Type     | Description
 | Name                                      | Type                                             | Description                                                                                                                                                                          | Default |
 | -----------                               | --------                                         | ------------------------------------------------------------------------                                                                                                             | ------- |
 | **internal**                              | _boolean_                                        | Whether the domain is used for internal (container-to-container) traffic.                                                                                                            | false   |
-| **router_group.guid**                     | _uuid_                                           | The desired router group guid. <br>_Note: cannot be used when `internal` is set to `true` or domain is scoped to an org._                       | null    |
+| **router_group.guid**                     | _uuid_                                           | **Experimental** - The desired router group guid. <br>_Note: cannot be used when `internal` is set to `true` or domain is scoped to an org._                                         | null    |
 | **supported_protocols**                   | _list of strings_                               | The protocols that are supported by the domain.                       |        |
-| **organization**                          | [_to-one relationship_](#to-one-relationships)   | A relationship to the organization the domain will be scoped to. <br>_Note: cannot be used when `internal` is set to `true` or domain is associated with a router group._           |         |
-| **shared_organizations**                  | [_to-many relationship_](#to-many-relationships) | A relationship to organizations the domain will be shared with. <br>_Note: cannot be used without an organization relationship_                                                     |         |
+| **organization**                          | [_to-one relationship_](#to-one-relationships)   | A relationship to the organization the domain will be scoped to. <br>_Note: cannot be used when `internal` is set to `true` or domain is associated with a router group._            |         |
+| **shared_organizations**                  | [_to-many relationship_](#to-many-relationships) | A relationship to organizations the domain will be shared with. <br>_Note: cannot be used without an organization relationship_                                                      |         |
 | **metadata.labels**                       | [_label object_](#labels)                        | Labels applied to the domain.                                                                                                                                                        |         |
 | **metadata.annotations**                  | [_annotation object_](#annotations)              | Annotations applied to the domain.                                                                                                                                                   |         |