Add note about the version in which GetoptError was added -- this can

bite people interested in 1.5.2 compatibility.

Doc/lib/libgetopt.tex

@@ -64,6 +64,9 @@ not require one will also cause this exception to be raised.  The
 attributes \member{msg} and \member{opt} give the error message and
 related option; if there is no specific option to which the exception
 relates, \member{opt} is an empty string.
+
+\versionchanged[Introduced \exception{GetoptError} as a synonym for
+                \exception{error}]{1.6}
 \end{excdesc}
 
 \begin{excdesc}{error}