Rename the parameters of atan2(), based on comments from Guido & Peter

A. Koren <pkoren@hex.net>.

Doc/lib/libmath.tex

@@ -22,8 +22,8 @@ Return the arc sine of \var{x}.
 Return the arc tangent of \var{x}.
 \end{funcdesc}
 
-\begin{funcdesc}{atan2}{x, y}
-Return \code{atan(\var{x} / \var{y})}.
+\begin{funcdesc}{atan2}{y, x}
+Return \code{atan(\var{y} / \var{x})}.
 \end{funcdesc}
 
 \begin{funcdesc}{ceil}{x}