Remove isinstance check from FunctionTool (#3987)

[gziz]

Why are these changes needed?

The following PR includes:

• Removed the isinstance check from the FunctionTool class, allowing users to wrap functions that return lists or other subscripted generics, which was previously not possible. I have added a test for a function that returns a list; this would have failed with the previous implementation (see image 1).
• Added a FunctionTool docstring, including an example (see image 2).
• Expanded the documentation for Custom Function Tools to emphasize the importance of providing a clear description and type annotations when wrapping a function (see image 3).

Image 1

Image 2

Image 3

Related issue number

Closes #3987

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.