In the original disambiguation fix for `GenericValue::operator[]` (#170),
the documentation has been missing, which led to quite badly rendered
Doxygen pages.
During a cleanup, I've realized that a much simpler disambiguation is
possible:
````cpp
GenericValue& operator[](SizeType idx); // array
template <typename T>
GenericValue& operator[](T* name); // object
````
This approach works, as non-template functions are preferred over
template functions.
In order to improve the error messages, the pointer type is restricted
to `(const) Ch`.
Update `tutorial.md` to drop the ambiguity warning.
e8445e0bbc