[qty.unit.types] Specify get_quantity_spec(AssociatedUnit)

docs/api_reference/src/quantities.tex

@@ -2584,7 +2584,7 @@
 \begin{itemdescr}
 \pnum
 \returns
-TBD.
+\tcode{\exposidnc{get-associated-quantity}(u)}.
 \end{itemdescr}
 
 \begin{itemdecl}