docs: update handoff tool documentation with missing API features

Align the handoff tool docs with the current API schema by adding
documentation for squad destinations, tool messages, rejection plans,
the userAndAssistantMessages context plan, variable extraction aliases,
variable access patterns, and assistant destination overrides.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Vapi Tasker