Document generic type parameters to FunctionElement and GenericFunction (#9079)

Document generic type parameters to FunctionElement and GenericFunction (#9079)

* Document type parameters to FunctionElement

Add a note to FunctionElement which indicates that the type is a
typing.Generic class and points at GenericFunction examples for a
specific example usage.

A minimal reference is made to type checkers and IDEs as use-cases in
order to try to contextualize this as an optional feature which
supports particular use cases.

Append to the GenericFunction examples a case which uses `DateTime`
but also includes the generic type parameter (`datetime.datetime`).

* Fix type annotated function usage example