docs: clarify ToolsToFinalOutputResult docstrings

[MuhammedSuhaib]

• Reworded is_final_output docstring for clarity.
• Made rules for final_output explicit:
  • Must be non-None and match output_type when is_final_output=True.
  • Must be None when is_final_output=False.
• No functional changes, docs only.

[seratch]

Thanks for the suggestion, but I don't think the current one is so bad. Let me skip the change this time.