chore: resolve some doc warnings

[polvalente]

closes #249

[wingyplus]

@polvalente you may need to add start_server to this line https://github.com/elixir-grpc/grpc/blob/master/interop/script/run.exs#L17 since interop test does not starting a server

[polvalente]

@polvalente you may need to add start_server to this line https://github.com/elixir-grpc/grpc/blob/master/interop/script/run.exs#L17 since interop test does not starting a server

The problem was that nil or ... doesn't work. It should have been opts[:start_server] || ..., but I refactored into a cond instead

[wingyplus]

LGTM