V3: Document service usage events

The API documentation includes an example service usage event.

`GET /v3/usage_events/:guid`

Usage events are a union of app usage events and service usage events.
Our documentation included an example of an app usage event but not a
service usage event.

[finishes #172985853]

Co-authored-by: Reid Mitchell <rmitchell@pivotal.io>
Co-authored-by: Brian Cunnie <bcunnie@pivotal.io>

Author: reidmit

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

@@ -1,8 +1,16 @@
-<% content_for :single_usage_event do %>
+<% content_for :single_app_usage_event do %>
 {
   "guid": "a595fe2f-01ff-4965-a50c-290258ab8582",
   "created_at": "2020-05-28T16:41:23Z",
   "updated_at": "2020-05-28T16:41:26Z",
   "type": "app"
 }
 <% end %>
+<% content_for :single_service_usage_event do %>
+{
+  "guid": "c9976002-96f4-435a-888e-db1e1178df62",
+  "created_at": "2020-05-28T12:34:56Z",
+  "updated_at": "2020-05-28T12:34:56Z",
+  "type": "service"
+}
+<% end %>

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

@@ -1,10 +1,16 @@
 ### The usage event object
 
 ```
-Example Usage Event object
+Example of an App Usage Event object
 ```
 ```json
-<%= yield_content :single_usage_event %>
+<%= yield_content :single_app_usage_event %>
+```
+```
+Example of a Service Usage Event object
+```
+```json
+<%= yield_content :single_service_usage_event %>
 ```
 
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