Clarify that date_to_xmlschema returns an ISO 8601 string

Adds a helpful little reminder to the docs that `date_to_xmlschema` returns an ISO 8601 string.

![screen shot 2013-09-01 at 12 07 35 pm](https://f.cloud.github.com/assets/1420926/1064364/0d5bddd4-1329-11e3-83bd-4a630999707f.png)