Add index entry for reference to the math module.

Doc/lib/libcmath.tex

@@ -82,10 +82,10 @@ The mathematical constant \emph{e}, as a real.
 \end{datadesc}
 
 Note that the selection of functions is similar, but not identical, to
-that in module \code{math}.  The reason for having two modules is,
-that some users aren't interested in complex numbers, and perhaps
-don't even know what they are.  They would rather have \code{math.sqrt(-1)}
-raise an exception than return a complex number.  Also note that the
-functions defined in \code{cmath} always return a complex number, even
-if the answer can be expressed as a real number (in which case the
-complex number has an imaginary part of zero).
+that in module \code{math}\refbimodindex{math}.  The reason for having
+two modules is, that some users aren't interested in complex numbers,
+and perhaps don't even know what they are.  They would rather have
+\code{math.sqrt(-1)} raise an exception than return a complex number.
+Also note that the functions defined in \code{cmath} always return a
+complex number, even if the answer can be expressed as a real number
+(in which case the complex number has an imaginary part of zero).

Doc/libcmath.tex

@@ -82,10 +82,10 @@ The mathematical constant \emph{e}, as a real.
 \end{datadesc}
 
 Note that the selection of functions is similar, but not identical, to
-that in module \code{math}.  The reason for having two modules is,
-that some users aren't interested in complex numbers, and perhaps
-don't even know what they are.  They would rather have \code{math.sqrt(-1)}
-raise an exception than return a complex number.  Also note that the
-functions defined in \code{cmath} always return a complex number, even
-if the answer can be expressed as a real number (in which case the
-complex number has an imaginary part of zero).
+that in module \code{math}\refbimodindex{math}.  The reason for having
+two modules is, that some users aren't interested in complex numbers,
+and perhaps don't even know what they are.  They would rather have
+\code{math.sqrt(-1)} raise an exception than return a complex number.
+Also note that the functions defined in \code{cmath} always return a
+complex number, even if the answer can be expressed as a real number
+(in which case the complex number has an imaginary part of zero).