Document host_supported and device_supported

Test: none
Change-Id: I151fa044a61a0fc993ce4ea672fda762c15025cc
diff --git a/android/module.go b/android/module.go
index 556c73d..688d8b2 100644
--- a/android/module.go
+++ b/android/module.go
@@ -280,7 +280,10 @@
 }
 
 type hostAndDeviceProperties struct {
-	Host_supported   *bool
+	// If set to true, build a variant of the module for the host.  Defaults to false.
+	Host_supported *bool
+
+	// If set to true, build a variant of the module for the device.  Defaults to true.
 	Device_supported *bool
 }