Extend a code comment

siunitx-quantity.dtx

@@ -384,10 +384,11 @@
 %   }
 %   For printing a single part of a quantity. This is needed for compound
 %   quantities and so is public: that's also the reason for passing both
-%   argument explicitly. The lazy test here is looking for the case where
-%   a |1| has been inserted at the start of a format unit \emph{and} we
-%   have some other number to print: the |1| is then removed and there is
-%   no space inserted.
+%   argument explicitly. The semantics here require that the penalty and
+%   spacing are always applied: the number argument may be empty. The lazy test
+%   here is looking for the case where a |1| has been inserted at the start of
+%   a format unit \emph{and} we have some other number to print: the |1| is
+%   then removed and there is no space inserted.
 %    \begin{macrocode}
 \cs_new_protected:Npn \siunitx_quantity_print:nn #1#2
   {