Clearer schema descriptions across Broker, Auth, and Market Data

Broker API

Documentation improvements to the activity and funding event schemas. Response shapes and accepted values are unchanged.

• Adds descriptions to the shared non-trade activity schemas, including CommonNTAActivityV2 ("Shared fields for non-trade (NTA) activity records.") and CommonJournalActivityV2 ("Shared fields for journal non-trade activity records.").
• Simplifies the activity detail and options activity schemas by flattening redundant schema combinators; the per-activity variants list is unchanged.
• Clarifies the funding status event description: it covers bank relationships, bank wires, transfers, and funding wallets.

Affected Endpoints

• GET /v2/events/funding/status
• GET /v2beta1/events/activities
• GET /v2beta1/accounts/{account_id}/events/activities/{event_id}

Market Data API

Simplifies the sort query parameter definition on the corporate actions endpoint. Accepted values and behavior are unchanged.

Affected Endpoints

• GET /v1/corporate-actions

Auth API

The Auth API description now includes a top-level service description: "Authenticate clients and manage access to Alpaca APIs".