Only document <file>.xreadlines() once; added version annotation.

This closes SF bug #417943.

Doc/lib/libstdtypes.tex

@@ -1192,7 +1192,10 @@ Files have the following methods:
 \end{methoddesc}
 
 \begin{methoddesc}[file]{xreadlines}{}
-  Equivalent to \function{xreadlines.xreadlines(file)}.\refstmodindex{xreadlines}
+  Equivalent to
+  \function{xreadlines.xreadlines(\var{file})}.\refstmodindex{xreadlines}
+  (See the \refmodule{xreadlines} module for more information.)
+  \versionadded{2.1}
 \end{methoddesc}
 
 \begin{methoddesc}[file]{seek}{offset\optional{, whence}}
@@ -1233,12 +1236,6 @@ Files have the following methods:
   \method{writelines()} does not add line separators.)
 \end{methoddesc}
 
-\begin{methoddesc}[file]{xreadlines}{}
-  Equivalent to
-  \function{xreadlines.xreadlines(\var{file})}.\refstmodindex{xreadlines}
-  (See the \refmodule{xreadlines} module for more information.)
-\end{methoddesc}
-
 
 File objects also offer a number of other interesting attributes.
 These are not required for file-like objects, but should be