Portfolio history examples now document cashflow data

Expands the Get Account Portfolio History documentation with examples that show cashflow data in the response:

• Adds response examples illustrating the cashflow object, which breaks down cash movements by type (for example DIV and CSD).
• Documents the cashflow_types query parameter in more detail.

The response shape is unchanged.

Broker API

• GET /v1/trading/accounts/{account_id}/account/portfolio/history