Add "version added" for getpgid

Doc/lib/libos.tex

@@ -148,6 +148,7 @@ Availability: \UNIX.
 Return the process group id of the process with process id \var{pid}.
 If \var{pid} is 0, the process group id of the current process is
 returned. Availability: \UNIX.
+\versionadded{2.3}
 \end{funcdesc}
 
 \begin{funcdesc}{getpgrp}{}