Add some minimal doc for get_completer added for patch 676342

Doc/lib/libreadline.tex

@@ -80,6 +80,11 @@ until it returns a non-string value.  It should return the next
 possible completion starting with \var{text}.
 \end{funcdesc}
 
+\begin{funcdesc}{get_completer}{}
+Get the completer function.
+\versionadded{2.3}
+\end{funcdesc}
+
 \begin{funcdesc}{get_begidx}{}
 Get the beginning index of the readline tab-completion scope.
 \end{funcdesc}