Update HostAndDevice* documentation

Clarify that HostAndDeviceSupported builds _only_ Device by default,
and add information on how to built host variations.
For HostAndDeviceDefault, clarify how to disable different variants.

Change-Id: I20e30175ddf79038117f1df166d9b994237b27f5
diff --git a/android/module.go b/android/module.go
index b571f15..cc03418 100644
--- a/android/module.go
+++ b/android/module.go
@@ -986,10 +986,13 @@
 	// Device is built by default. Host and HostCross are not supported.
 	DeviceSupported = deviceSupported | deviceDefault
 
-	// Device is built by default. Host and HostCross are supported.
+	// By default, _only_ device variant is built. Device variant can be disabled with `device_supported: false`
+    // Host and HostCross are disabled by default and can be enabled with `host_supported: true`
 	HostAndDeviceSupported = hostSupported | hostCrossSupported | deviceSupported | deviceDefault
 
 	// Host, HostCross, and Device are built by default.
+    // Building Device can be disabled with `device_supported: false`
+    // Building Host and HostCross can be disabled with `host_supported: false`
 	HostAndDeviceDefault = hostSupported | hostCrossSupported | hostDefault |
 		deviceSupported | deviceDefault