docs(agents): document realtime capabilities

[shizhigu]

Summary

• Add docstrings for the previously undocumented RealtimeCapabilities fields.
• Clarify that realtime-backed session.say() ignores add_to_chat_ctx=False and still adds the message to chat context.
• Clarify that manual_function_calls means function call items already in chat context can be resumed.
• Keep the change documentation-only: no defaults, provider declarations, or runtime behavior changed.

Closes #5565.

Verification

• Checked each capability docstring against the corresponding capabilities.<field> usage in agent_activity.py and agent.py.
• make check
• git diff --check

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.