doc: note util.isError() @@toStringTag limitations

util.isError() is the only remaining util.is*() method that
depends on Object.prototype.toString() behavior. This commit
notes the limitations of isError() related to @@toStringTag.

Refs: https://github.com/nodejs/node/pull/2201
PR-URL: https://github.com/nodejs/node/pull/5414


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>