"REORG" activity type clarified as worthless removal

Clarifies the documentation for the REORG activity type to distinguish it from REO.

• REORG is now described as a worthless removal corporate action, its only supported case (sub-type WRM).
• REO remains the type for broader reorganization corporate actions, with sub-types REOS, REOC, and REOSC.
• Description-only change; returned values and behavior are unchanged.

Broker API

• GET /v1/accounts/activities
• GET /v1/accounts/activities/{activity_type}

Trading API

• GET /v2/account/activities
• GET /v2/account/activities/{activity_type}