[ffigen] Document C++ best practices

[goderbauer]

C++ doesn't have a stable ABI, so direct interop isn't possible. Instead you should make a wrapper C API using extern "C".

There's a proposal to write a tool to autogenerate this wrapper, which we hope someone from the community would like to take on: #2644

In any case, we should document how to handle C++ somewhere in the ffigen docs.

cc @liamappelbe