docs: clarify FunctionTool parameter handling and add examples

[lavinigam-gcp]

This PR addresses outdated and incorrect documentation for FunctionTool parameter handling, closing issues #473, #481, and #210.

The existing documentation incorrectly advises against using default values for parameters, a feature that is fully supported by the ADK framework. This has caused confusion and limited developers' ability to define flexible tools.

This change:

• Removes the inaccurate statement about default parameters.
• Completely overhauls the 'Parameters' section in function-tools.md into a new, structured guide on defining function signatures.
• Provides clear, distinct code examples for required parameters, optional parameters with default values, and optional parameters using typing.Optional.
• Clarifies that variadic arguments (*args, **kwargs) are ignored for schema generation.
• Briefly explains how ADK inspects function signatures to generate the LLM tool schema, adding clarity to the automatic wrapping process.

This comprehensive update will provide developers with accurate guidance, prevent common errors, and enable them to leverage the full capabilities of FunctionTool.

[Hereforlolz]

@lavinigam-gcp This is awesome! Just tested it out and everything works like a charm.
I tested and threw a bunch of different function signatures and they all worked. Thank you for taking time to rewrite this whole section and putting effort into this!