Be more explicit in schema docs about the format of statementDate

[stevenday]

The example data looks like we're intending this to be ISO8601 like YYYY-MM-DD, but given that we have other dates which allow approximations (see #162) it feels like these should be more explicit.

[timgdavies]

Linked to #12 this might require some documentation about handling fuzzy dates (from both data producer and user) perspective if this appears to be a relevant issue to address right now.

[kd-ods]

#387 Looks at issues like this across the schema.

[kd-ods]

Closing in favour of #162 and related work.