Dexterity TTW XML Editor needs documentation

[jensens]

see plone/plone.app.dexterity#123 open here, close over there

[polyester]

note: any changes will have to be done at https://github.com/plone/plone.app.dexterity/blob/master/docs/reference/dexterity-xml.rst; docs.plone.org pulls in that as external documentation.