Documentation string for @iso-date attributes was lost from v2.01

[dalepotter]

See differences between how iati-activities/iati-activity/activity-date/@iso-date is defined at:

• Version 1.05: https://github.com/IATI/IATI-Schemas/blob/version-1.05/iati-activities-schema.xsd#L461-L468
• Version 2.01: https://github.com/IATI/IATI-Schemas/blob/version-2.01/iati-activities-schema.xsd#L664

This also applies to other instances of @iso-date attributes from v2.01.

[andylolz]

Some explanation provided in the message of the relevant commit (099156bb) and issue (#177).