v3 docs: Fix app/service usage docs

- Remove deleted `type` field on app usage events
- Improve description of service usage events

[finishes #173185318]
[finishes #173185584]

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

Author: reidmit

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

@@ -10,7 +10,6 @@ Example App Usage Event object
 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)
 **state.current** | _string_ or `null` | Current state of the app that this event pertains to, if applicable.

docs/v3/source/includes/experimental_resources/service_usage_events/_header.md

@@ -1,6 +1,9 @@
 ## Service Usage Events
 
-Service usage events are a record of changes in the usage services.
+Service usage events are a record of changes in the usage of services. For
+example, service usage events are recorded when a service instance is created,
+updated, and deleted. They may contain information about the service instance in
+question and the associated space, service broker, and service offering.
 
 Usage events are typically used by billing and chargeback applications.