Return F# signature text for C# override.

[nojaf]

For a symbol for WriteJson in override this.WriteJson(writer, value, serializer) = failwith "todo"

GetValSignatureText would return

JsonConverter.WriteJson(writer: obj, value: obj, serializer: obj) : unit,

after this PR it returns override WriteJson: writer: obj * value: obj * serializer: obj -> unit

I'll need to see if CI agrees with the override keyword change.

[psfinaki]

Awesome!