Fix and Document parallel_tool_calls Attribute in ModelSettings (#763)

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](https://platform.openai.com/docs/guides/function-calling?api-mode=responses#parallel-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`:
```python
Runner.run(agent, input, run_config=RunConfig(model_settings=ModelSettings(parallel_tool_calls=False)))
```
* Example for agent initialization:
```python
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.

Author: Rehan-Ul-Haq

src/agents/model_settings.py

@@ -37,8 +37,13 @@ class ModelSettings:
     """The tool choice to use when calling the model."""
 
     parallel_tool_calls: bool | None = None
-    """Whether to use parallel tool calls when calling the model.
-    Defaults to False if not provided."""
+    """Controls whether the model can make multiple parallel tool calls in a single turn.
+    If not provided (i.e., set to None), this behavior defers to the underlying
+    model provider's default. For most current providers (e.g., OpenAI), this typically
+    means parallel tool calls are enabled (True).
+    Set to True to explicitly enable parallel tool calls, or False to restrict the
+    model to at most one tool call per turn.
+    """
 
     truncation: Literal["auto", "disabled"] | None = None
     """The truncation strategy to use when calling the model."""