Add documentation about document field default values

When setting static default values in schemas, it's not always easy to know the syntax.
It's quite straight-forward for String, Boolean or Integer fields, but Date fields have to follow the W3C format (2011-10-23T12:00:00.00Z for instance, to check) and multi-valued fields also accept default values (separated by spaces to make the list).

There is already some documentation about how to set dynamic default values (see http://doc.nuxeo.com/x/QIbZ and http://doc.nuxeo.com/x/bQvF) but i could not find any about static fields.

When this is done, these docs should be linked (and it would be useful to reference the new one on the schema help in Studio)