Add a versionadded tag for traceback.format_exc()

Doc/lib/libtraceback.tex

@@ -51,6 +51,7 @@ variables.)
 \begin{funcdesc}{format_exc}{\optional{limit\optional{, file}}}
 This is like \code{print_exc(\var{limit})} but returns a string
 instead of printing to a file.
+\versionadded{2.4}
 \end{funcdesc}
 
 \begin{funcdesc}{print_last}{\optional{limit\optional{, file}}}