add missing version identification

Doc/lib/libgetopt.tex

@@ -65,6 +65,8 @@ encountered.
 If the first character of the option string is `+', or if the
 environment variable POSIXLY_CORRECT is set, then option processing
 stops as soon as a non-option argument is encountered.
+
+\versionadded{2.3}
 \end{funcdesc}
 
 \begin{excdesc}{GetoptError}