Restore mistakenly removed comment

src/core/include/units/magnitude.h

@@ -458,6 +458,12 @@ template<std::intmax_t N>
 static constexpr auto prime_factorization_v = prime_factorization<N>::value;
 } // namespace detail
 
+/**
+ * @brief  Convert any positive integer to a Magnitude.
+ *
+ * This will be the main way end users create Magnitudes.  They should rarely (if ever) create a magnitude<...> by
+ * manually adding base powers.
+ */
 template<ratio R>
   requires (R.num > 0)
 constexpr Magnitude auto as_magnitude() {