Document unused function in HIDL

This costs us very little. However, there aren't many good tools to
easily detect this kind of thing. So, documenting that it can be
removed, in case it can be in the course of further cleanup.

Fixes: 132385873
Test: N/A
Change-Id: Ifc0c2a9aa88946abd74e2d1c2e7c6c2f4702df18
diff --git a/transport/ServiceManagement.cpp b/transport/ServiceManagement.cpp
index d7faa6d..a69cf3e 100644
--- a/transport/ServiceManagement.cpp
+++ b/transport/ServiceManagement.cpp
@@ -192,6 +192,7 @@
     tryShortenProcessName(descriptor);
 }
 
+// only used by prebuilts - should be able to remove
 void onRegistration(const std::string& packageName, const std::string& interfaceName,
                     const std::string& instanceName) {
     return onRegistrationImpl(packageName + "::" + interfaceName, instanceName);