Make warning more dire

include/grpc/grpc_security.h

@@ -146,6 +146,10 @@ GRPCAPI void grpc_channel_credentials_release(grpc_channel_credentials* creds);
    If specified, the supplied call credentials object will be attached to the
    returned channel credentials object. The call_credentials object must remain
    valid throughout the lifetime of the returned grpc_channel_credentials object.
+   It is expected that the call credentials object was generated according to
+   the Application Default Credentials mechanism and asserts the identity of
+   default service account of the machine. Supplying any other sort of call
+   credential may result in RPCs suddenly and unexpectedly failing.
 
    If nullptr is supplied, the returned call credentials object will use a call
    credentials object based on the default service account of the VM.