Add versionadded. Remove duplicate "to"

Doc/lib/libthread.tex

@@ -45,7 +45,8 @@ then the thread exits (but other threads continue to run).
 
 \begin{funcdesc}{interrupt_main}{}
 Raise a KeyboardInterrupt in the main thread.  A subthread can use this
-function to to interrupt the main thread.
+function to interrupt the main thread.
+\versionadded{2.3}
 \end{funcdesc}
 
 \begin{funcdesc}{exit}{}