[qty.expr.temp.algo] Clarify position of per

docs/api_reference/src/quantities.tex

@@ -889,7 +889,7 @@
 \item
 \tcode{OneType} is the neutral element\irefiev{102-01-19} of the operation, and
 \item
-\tcode{Pred} is a total order for the mapped types.
+\tcode{Pred} is a total order for the mapped types other than a specialization of \tcode{per}.
 \end{itemize}
 
 \pnum
@@ -970,18 +970,22 @@
 \item
 Otherwise, if $y$ is a symbolic constant, returns $y$.
 \item
-Otherwise, returns \tcode{To<$y$>}
-after applying the following mappings and
-sorting the template argument lists of \tcode{To} and \tcode{per} according to \tcode{Pred}:
+Otherwise, first applies the following mappings to the terms of $y$:
 \begin{itemize}
 \item
 $x^{n/d}$ is mapped to \tcode{power<$x$, $n$, $d$>}, and
-$x^{n}$ is mapped to \tcode{power<$x$, $n$>},
+$x^{n}$ is mapped to \tcode{power<$x$, $n$>}, and
 \item
-$1$ is mapped to \tcode{OneType\{\}}, and
-\item
-a denominator $x$ is mapped to \tcode{per<$x$>}.
+$1$ is mapped to \tcode{OneType\{\}}.
 \end{itemize}
+\item
+Then, a denominator $x$ of $y$ (if any) is mapped to \tcode{per<$x$>}.
+\item
+Then, sorts $y$ without \tcode{per} (if any) and
+the template arguments of \tcode{per} (if any)
+according to \tcode{Pred}.
+\item
+Finally, returns \tcode{To<$y$>}, where \tcode{per} (if any) is the last argument.
 \end{itemize}
 
 \pnum