status: clarify FromError docstring (#4880)

status/status.go

@@ -73,11 +73,16 @@ func FromProto(s *spb.Status) *Status {
 	return status.FromProto(s)
 }
 
-// FromError returns a Status representing err if it was produced by this
-// package or has a method `GRPCStatus() *Status`.
-// If err is nil, a Status is returned with codes.OK and no message.
-// Otherwise, ok is false and a Status is returned with codes.Unknown and
-// the original error message.
+// FromError returns a Status representation of err.
+//
+// - If err was produced by this package or implements the method `GRPCStatus()
+//   *Status`, the appropriate Status is returned.
+//
+// - If err is nil, a Status is returned with codes.OK and no message.
+//
+// - Otherwise, err is an error not compatible with this package.  In this
+//   case, a Status is returned with codes.Unknown and err's Error() message,
+//   and ok is false.
 func FromError(err error) (s *Status, ok bool) {
 	if err == nil {
 		return nil, true