@ref definitions explicitly mention that they are optional

[hayfield]

The definitions for some ref attributes specify that they are optional.

• https://github.com/IATI/IATI-Schemas/blob/version-2.02/iati-organisations-schema.xsd#L220
• https://github.com/IATI/IATI-Schemas/blob/version-2.02/iati-organisations-schema.xsd#L363
• https://github.com/IATI/IATI-Schemas/blob/version-2.02/iati-organisations-schema.xsd#L513
• https://github.com/IATI/IATI-Schemas/blob/version-2.02/iati-organisations-schema.xsd#L644
• https://github.com/IATI/IATI-Schemas/blob/version-2.02/iati-organisations-schema.xsd#L745

The definitions of other optional attributes do not specify that they are optional.

This should be defined programmatically, as required attributes are, so that it is consistent across all attributes.