Customize help text and add ServerWithDoc

[bufdev]

For example with https://github.com/pluginrpc/pluginrpc-go/blob/main/internal/example/cmd/echo-plugin/main.go:

An example plugin that implements the EchoService.

Commands:

  echo error
  echo request
  /pluginrpc.example.v1.EchoService/EchoList

Flags:

      --format string   The format to use for requests, responses, and specs. Must be one of ["binary", "json"]. (default "binary")
      --protocol        Print the protocol to stdout and exit.
      --spec            Print the spec to stdout in the specified format and exit.
  -h, --help            Show this help.