Add ref to errno module.

Doc/lib/libposix.tex

@@ -58,6 +58,8 @@ string value is \code{'posix.error'}.  The accompanying value is a
 pair containing the numeric error code from \code{errno} and the
 corresponding string, as would be printed by the C function
 \code{perror()}.
+See the module \module{errno}\refbimodindex{errno}, which contains
+names for the error codes defined by the underlying operating system.
 \end{excdesc}
 
 It defines the following functions and constants:

Doc/libposix.tex

@@ -58,6 +58,8 @@ string value is \code{'posix.error'}.  The accompanying value is a
 pair containing the numeric error code from \code{errno} and the
 corresponding string, as would be printed by the C function
 \code{perror()}.
+See the module \module{errno}\refbimodindex{errno}, which contains
+names for the error codes defined by the underlying operating system.
 \end{excdesc}
 
 It defines the following functions and constants: