Incomplete and inconsistent documentation for FunctionTool parameter handling in Google ADK

[Hereforlolz]

Describe the bug
The official documentation for FunctionTool in Google ADK lacks clarity on how to define functions with required positional, optional, and variadic parameters for wrapping. This causes confusion and runtime errors, as seen in issues like #1592

To Reproduce
Steps to reproduce the behavior:

1. Follow current docs to define a function with a mix of required and optional parameters.
2. Initialize FunctionTool with this function.
3. Encounter unexpected errors or unclear guidance on how parameters are handled.

Expected behavior
Clear documentation with examples on supported function signatures, including required, optional, and variadic parameters.

Actual Behaviour
Current docs omit important details, leading to developer confusion and errors.

Desktop (please complete the following information):

• OS: [e.g. iOS]: Windows 11 Home
• ADK version(pip show google-adk): 1.2.1

Additional context
Improved docs would reduce friction and prevent issues like #1592.