Update docstring about tool messages in prepare_multimodal_messages

[albertvillanova]

Update docstring about tool messages in prepare_multimodal_messages.

Note that after #5440, messages with "tool" role are prepared for multimodal, but the docstring was not updated.

This PR includes a minor documentation update to the prepare_multimodal_messages function. The change removes a no longer applicable note about handling "tool_calls" and "tool" role in the function's docstring.

Follow-up to:

• Simplify _get_tool_suffix_ids #5440

Changes

• Documentation update:
  • Removed a docstring note stating that "tool_calls" in "assistant" turns and "tool" roles with "content" and "name" are left as-is, as these do not require special handling for multimodal data. (trl/data_utils.py)

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Note

Low Risk
Docstring-only change with no runtime behavior impact.

Overview
Updates the prepare_multimodal_messages docstring in trl/data_utils.py by removing an outdated note claiming tool_calls/tool role messages are left untouched during multimodal preparation.

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