*: add/fix godoc comments to satisfy golint

Fixes things like:
 * exported type/method/function X should have comment or be unexported
 * comment on exported type/method/function X should be of the form "X ..."
   (with optional leading article)

Refs. #213.

pkg/smartcontract/param_context.go

@@ -51,6 +51,7 @@ func (pt ParamType) String() string {
 	}
 }
 
+// MarshalJSON implements the json.Marshaler interface.
 func (pt ParamType) MarshalJSON() ([]byte, error) {
 	return []byte(`"` + pt.String() + `"`), nil
 }