`any_invocable`: Add public documentation for undefined behavior when invoking an empty AnyInvocable

This is currently documented in `internal/any_invocable.h`, but not in the public API.
For example, `std::function` [documents](https://en.cppreference.com/w/cpp/utility/functional/function) the fact that invoking an empty instance throws `std::bad_function_call`.

PiperOrigin-RevId: 631621604
Change-Id: I6b886a805ffa0e7aaef5f6971e1eafd14f94050c