Update BroadcastDispatcher docs to say not to call goAsync()

Shouldn't be needed within sysui as we're not worried about the process
being killed.

Change-Id: I14977770123c9b41fec1ac556a539cb8f09b9f2b

packages/SystemUI/docs/broadcasts.md

@@ -21,6 +21,7 @@ The dispatcher supports `BroadcastReceiver` dynamic subscriptions in the followi
 * The `IntentFilter` may or may not contain categories.
 * The `IntentFilter` **does not** contain data types, data schemes, data authorities or data paths.
 * The broadcast **is not** gated behind a permission.
+* The broadcast **is not** ordered and doesn't need to set result data.
 
 Additionally, the dispatcher supports the following:
 
@@ -107,3 +108,8 @@ fun unregisterReceiverForUser(BroadcastReceiver, UserHandle)
 ```
 
 Unregistering can be done even if the `BroadcastReceiver` has never been registered with `BroadcastDispatcher`. In that case, it is a No-Op.
+
+### A note on goAsync()
+
+If you're processing a broadcast in a background thread, you shouldn't call `goAsync()` and
+`finish()`. The system will keep sysui alive regardless, so it isn't needed.

packages/SystemUI/src/com/android/systemui/broadcast/BroadcastDispatcher.kt

@@ -60,6 +60,8 @@ private const val DEBUG = true
  * Use only for IntentFilters with actions and optionally categories. It does not support,
  * permissions, schemes, data types, data authorities or priority different than 0.
  * Cannot be used for getting sticky broadcasts (either as return of registering or as re-delivery).
+ * Broadcast handling may be asynchronous *without* calling goAsync(), as it's running within sysui
+ * and doesn't need to worry about being killed.
  */
 open class BroadcastDispatcher constructor (
     private val context: Context,