make_iterator: accept args by value, not forwarding reference, so that passing lvalue iterators works

[oremanj]

This is a reasonable-looking way to expose std::map::equal_range() to Python:

    .def("equal_range", [](MapType& self, KeyType key) {
        auto range = self.equal_range(key);
        return nb::make_iterator(nb::type<MapType>(), "range_iterator", range.first, range.second);
    })

Unfortunately, it dangles: range.first has type MapType::iterator&, so deducing Iterator&& from it yields Iterator = MapType::iterator&, so a reference gets stored in the iterator state object. But that reference refers to the stack-allocated range object, which will be destroyed before the Python iterator is used. This PR changes make_iterator (and friends) arguments to accept their iterator and sentinel by value, to avoid this dangling risk. You can still get reference semantics by specifying the template arguments explicitly.

Also resolve ambiguity between the iterator and range overloads of make_iterator, and clean up the docs to reflect the iterator/sentinel split.

[wjakob]

That seems dangerous indeed. Thank you very much!