v3: Document app/service usage event data

Add documentation for all the data subfields on app and service usage
event objects.

[finishes #172985876]
[finishes #172985880]

Authored-by: Reid Mitchell <rmitchell@pivotal.io>

Author: reidmit

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

@@ -13,9 +13,52 @@ Example of a Service Usage Event object
 <%= yield_content :single_service_usage_event %>
 ```
 
+There two types of usage events: app usage events and service usage events. They are distinguished by their `type` field and by the fields displayed in their `data`.
+
+#### App usage events
+
+Name | Type | Description
+---- | ---- | -----------
+**guid** | _uuid_ | Unique identifier for the event.
+**type** | _string_ | The type of the event, always "app" for app usage events
+**created_at** | _datetime_ | The time with zone when the event occurred
+**updated_at** | _datetime_ | Identical to `created_at` (events are created, never updated)
+**data.state.current** | _string_ or `null` | Current state of the app that this event pertains to, if applicable.
+**data.state.previous** | _string_ or `null` | Previous state of the app that this event pertains to, if applicable.
+**data.app.guid** | _string_ or `null` | Unique identifier of the app that this event pertains to, if applicable.
+**data.app.name** | _string_ or `null` | Name of the app that this event pertains to, if applicable.
+**data.process.guid** | _string_ or `null` | Unique identifier of the process that this event pertains to, if applicable.
+**data.process.type** | _string_ or `null` | Type of the process that this event pertains to, if applicable.
+**data.space.guid** | _string_ or `null` | Unique identifier of the space that this event pertains to, if applicable.
+**data.space.name** | _string_ or `null` | Name of the space that this event pertains to, if applicable.
+**data.organization.guid** | _string_ or `null` | Unique identifier of the org that this event pertains to, if applicable.
+**data.buildpack.guid** | _string_ or `null` | Unique identifier of the buildpack that this event pertains to, if applicable.
+**data.buildpack.name** | _string_ or `null` | Name of the buildpack that this event pertains to, if applicable.
+**data.task.guid** | _string_ or `null` | Unique identifier of the task that this event pertains to, if applicable.
+**data.task.name** | _string_ or `null` | Name of the task that this event pertains to, if applicable.
+**data.memory_in_mb_per_instance.current** | _integer_ or `null` | Current memory in MB of the app that this event pertains to, if applicable.
+**data.memory_in_mb_per_instance.previous** | _integer_ or `null` | Previous memory in MB of the app that this event pertains to, if applicable.
+**data.instance_count.current** | _integer_ or `null` | Current instance count of the app that this event pertains to, if applicable.
+**data.instance_count.previous** | _integer_ or `null` | Previous instance count of the app that this event pertains to, if applicable.
+
+#### Service usage events
+
 Name | Type | Description
 ---- | ---- | -----------
 **guid** | _uuid_ | Unique identifier for the event.
-**type** | _string_ | The type of the event, either "app" (application event) or "service" (service event)
+**type** | _string_ | The type of the event, always "service" for service usage events
 **created_at** | _datetime_ | The time with zone when the event occurred
 **updated_at** | _datetime_ | Identical to `created_at` (events are created, never updated)
+**data.state** | _string_ or `null` | Current state of the service that this event pertains to, if applicable.
+**data.space.guid** | _string_ or `null` | Unique identifier of the space that this event pertains to, if applicable.
+**data.space.name** | _string_ or `null` | Name of the space that this event pertains to, if applicable.
+**data.organization.guid** | _string_ or `null` | Unique identifier of the org that this event pertains to, if applicable.
+**data.service_instance.guid** | _string_ or `null` | Unique identifier of the service instance that this event pertains to, if applicable.
+**data.service_instance.name** | _string_ or `null` | Name of the service instance that this event pertains to, if applicable.
+**data.service_instance.type** | _string_ or `null` | Type of the service instance that this event pertains to, if applicable.
+**data.service_plan.guid** | _string_ or `null` | Unique identifier of the service plan that this event pertains to, if applicable.
+**data.service_plan.name** | _string_ or `null` | Name of the service plan that this event pertains to, if applicable.
+**data.service_offering.guid** | _string_ or `null` | Unique identifier of the service offering that this event pertains to, if applicable.
+**data.service_offering.name** | _string_ or `null` | Name of the service offering that this event pertains to, if applicable.
+**data.service_broker.guid** | _string_ or `null` | Unique identifier of the service broker that this event pertains to, if applicable.
+**data.service_broker.name** | _string_ or `null` | Name of the service broker that this event pertains to, if applicable.