Closes #762

Description

This PR updates the docstring for the parallel_tool_calls attribute in the ModelSettings dataclass to accurately reflect its default behavior.. The previous docstring incorrectly stated that the default of False, while the actual behavior is dependent on the underlying model provider's default.

As noted in OpenAI's (here refers as model provider) Function Calling documentation, "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, when 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 defers to the model provider's default behavior for parallel tool calls. This is typically True for most current providers, but it's important to acknowledge that this isn't a fixed default within our codebase.

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

Key changes:

• Clarified the default behavior of parallel_tool_calls: Instead of stating a fixed default, the docstring now accurately reflects that the behavior defaults to whatever the model provider does when the attribute is None.
• 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 (Tested with openai models).
• 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.