Closes #762

Description

This PR updates the docstring for the parallel_tool_calls attribute in the ModelSettings dataclass. The previous docstring incorrectly stated that the default value is False, while the actual default is True.
In OpenAI docs of section Function Calling, it is clearly mentioned that "The model may choose to call multiple functions in a single turn. You can prevent this by setting parallel_tool_calls to false, which ensures exactly zero or one tool is called."

Therefore, if the parallel_tool_calls attribute in the ModelSettings dataclass is set to None (i.e., parallel_tool_calls: bool | None = None), and this value is passed directly to the API without modification, it effectively behaves as if parallel_tool_calls is set to True.

The new docstring is formatted for automatic documentation generation and provides clear, accurate information for users and developers.

Key changes:

• Corrected the documented default value of parallel_tool_calls from False to True.
• Improved the docstring format for compatibility with documentation tools.
• Clarified the purpose and usage of the parallel_tool_calls attribute.

Testing:

• Explicitly set parallel_tool_calls=False in both the run_config of the Runner.run method and in the agent’s model_settings attribute.
• Example for Runner.run:

Runner.run(agent, input, run_config=RunConfig(model_settings=ModelSettings(parallel_tool_calls=False)))

• Example for agent initialization:

agent = Agent(..., model_settings=ModelSettings(parallel_tool_calls=False))

• Verified that when parallel_tool_calls=False, tools are called sequentially.
• Confirmed that by default (without setting the attribute), tools are called in parallel.
• Checked that the updated docstring renders correctly in the generated documentation.
• Ensured the default value in code matches the documentation.

Why this is important:

• Prevents confusion for users and developers regarding the default behavior of parallel_tool_calls.
• Ensures that generated documentation is accurate and up-to-date.
• Improves overall code quality and maintainability.