RPC Binder: document RpcServerTrusty design path

This design was recently copied to add RpcServerChre. In
order to make this more clear, that functionality should
move directly into RPC Binder, document directly in
the header.

Bug: 387305736
Test: N/A
Change-Id: Ieb6c20d500a81905787489ece6d7bc8ee0d89e7b