docs: clarify runaway team safeguards

[maxpetrusenkoagent]

Summary

• Document that AgentChat does not currently provide built-in capacity declarations for agents or teams
• Add practical safeguards for runaway multi-agent retries using existing termination conditions, external termination, custom termination, and bounded tool loops

Test Plan

• python3 -m json.tool python/docs/src/user-guide/agentchat-user-guide/tutorial/termination.ipynb >/tmp/termination.ipynb.jsoncheck
• Python check that the new notebook cell is present
• git diff --check

Refs #7321