libbinder_ndk: document association for dead binder

The AIBinder_associateClass method checks interfaceDescriptor and may
invoke an IPC. In this case, if it is called on a binder which is dead,
it will fail, which may be surprising.

Bug: 155793159
Test: N/A
Change-Id: I09b2a0c5346d254fbad46086d47e2385f55ec3a2