Document GetoptError and label error an alias.
diff --git a/Doc/lib/libgetopt.tex b/Doc/lib/libgetopt.tex
index 72824a7..1c17ab5 100644
--- a/Doc/lib/libgetopt.tex
+++ b/Doc/lib/libgetopt.tex
@@ -41,12 +41,19 @@
be mixed.
\end{funcdesc}
-\begin{excdesc}{error}
+\begin{excdesc}{GetoptError}
This is raised when an unrecognized option is found in the argument
list or when an option requiring an argument is given none.
The argument to the exception is a string indicating the cause of the
error. For long options, an argument given to an option which does
-not require one will also cause this exception to be raised.
+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.
+\end{excdesc}
+
+\begin{excdesc}{error}
+Alias for \exception{GetoptError}; for backward compatibility.
\end{excdesc}