protocol: document "sub" field in error list
protocol: document "sub" field in error list

rgrinberg/merlin,rgrinberg/merlin,dra27/merlin,dra27/merlin,dra27/merlin,ocaml/merlin,the-lambda-church/merlin,the-lambda-church/merlin,ocaml/merlin,rgrinberg/merlin,dra27/merlin,rgrinberg/merlin,ocaml/merlin,the-lambda-church/merlin,ocaml/merlin