[qty.unit.types] Improve named_unit example comment

Co-authored-by: Mateusz Pusz <[email protected]>

docs/api_reference/src/quantities.tex

@@ -2052,7 +2052,7 @@
 \pnum
 \begin{example}
 \begin{codeblock}
-// The first signature defines a base unit.
+// The first signature defines a base unit restricted to a kind of quantity.
 inline constexpr struct second final : named_unit<"s", kind_of<time>> {
 } second;