"foreign" and "special" on dividend activities are JSON strings

Clarifies that foreign and special on dividend activity details are serialized as the JSON strings "true" or "false", not JSON booleans.

• Expanded field descriptions on the CommonCDIVActivityV2 and DIVSPDActivityV2 schemas.
• Added an example of "false" for both fields.
• The wire format is unchanged; only the documented representation is now explicit.

Broker API

• GET /v2beta1/events/activities
• GET /v2beta1/accounts/{account_id}/events/activities/{event_id}

Trading API

• GET /v2beta1/events/activities