👋 Hello! Very minor issue here, but I was hitting a bit of confusion trying to figure out if I should be using --<lang>_opt or --<lang>_out to pass a custom flag to a protogen plugin.

The current info in the godoc specifies the latter, which I believe is legacy. Maybe the opt equivalent should be documented there instead/as well?

Code for reference:
https://github.com/protocolbuffers/protobuf-go/blob/32018e9a48fe1fb3addef57cd8b745ade54fc0d7/compiler/protogen/protogen.go#L128-L150

Thanks!