Warn about the correct usage of discrete_space (#325)

* Warn about the correct usage of

* Review from Yuuichi

include/ddc/discrete_space.hpp

@@ -194,6 +194,12 @@ std::enable_if_t<2 <= sizeof...(Args), std::tuple<Args...>> init_discrete_space(
     return detail::extract_after(std::move(a), std::index_sequence_for<Args...>());
 }
 
+/**
+ * @tparam DDim a discrete dimension
+ * @return the discrete space instance associated with `DDim`.
+ * This function must be called from a `KOKKOS_FUNCTION`.
+ * Call `ddc::host_discrete_space` for a host-only function instead.
+ */
 template <class DDim, class MemorySpace = DDC_CURRENT_KOKKOS_SPACE>
 KOKKOS_FORCEINLINE_FUNCTION detail::ddim_impl_t<DDim, MemorySpace> const& discrete_space()
 {