Document xreadlines() method. (Forgot to check this in before!)

Doc/lib/libstdtypes.tex

@@ -1112,6 +1112,10 @@ Files have the following methods:
   implemented, or cannot be implemented efficiently.
 \end{methoddesc}
 
+\begin{methoddesc}[file]{xreadlines}{}
+  Equivalent to \function{xreadlines.xreadlines(file)}.\refstmodindex{xreadlines}
+\end{methoddesc}
+
 \begin{methoddesc}[file]{seek}{offset\optional{, whence}}
   Set the file's current position, like \code{stdio}'s \cfunction{fseek()}.
   The \var{whence} argument is optional and defaults to \code{0}