docs(realtime): note tool schema strictness limits

[adityasingh2400]

Summary

• Clarify that realtime function tools receive SDK-generated schemas, but strict tool-schema enforcement is not the same as Responses API function tools.
• Recommend flat realtime tool argument shapes or validating richer JSON payloads inside the tool before side effects.

Test plan

• uv run mkdocs build
• git diff --check

Closes #1681.

[seratch]

I don't think using complex inputs for tools that could be called by realtime agents is a realistic use case, so I'd like to hold off adding this section.