docs(sdk): document MCP tool refresh

[Shimada666]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

• Document automatic MCP tool reconciliation after notifications/tools/list_changed.
• Document explicit conversation.refresh_mcp_tools() usage after an MCP server deployment or restart.
• Clarify that add/update/remove changes affect subsequent LLM requests without altering historical events.

Matching SDK PR: OpenHands/software-agent-sdk#4402.

Validation:

• git diff --check passed.
• Docs tests excluding the unrelated stale pricing-source test: 30 passed.
• The full docs suite reached 34 passed; two pricing tests error because their upstream OpenHands/OpenHands/main/openhands/utils/llm.py URL currently returns 404.