Incorrect documentation about new widget return value

[strangemonad]

https://github.com/streamlit/docs/blob/14f9d3ba9f35cf5334d1d7d8b2039d42e874a4e9/content/develop/concepts/architecture/widget-behavior.md?plain=1#L190C4-L190C55

This is only the default value if there's no widget session state. The return value is actually the value of the widget

• The new widget's value is assigned from session state if the key exists, otherwise the default value parameter
• The widget's value is returned form the function