Document warning categories.

Doc/lib/libexcs.tex

@@ -330,3 +330,29 @@ For class exceptions, \function{str()} returns only the message.
   zero.  The associated value is a string indicating the type of the
   operands and the operation.
 \end{excdesc}
+
+
+\setindexsubitem{(built-in warning category)}
+
+The following exceptions are used as warning categories; see the
+\module{warnings} module for more information.
+
+\begin{excdesc}{Warning}
+Base class for warning categories.
+\end{excdesc}
+
+\begin{excdesc}{UserWarning}
+Base class for warnings generated by user code.
+\end{excdesc}
+
+\begin{excdesc}{DeprecationWarning}
+Base class for warnings about deprecated features.
+\end{excdesc}
+
+\begin{excdesc}{SyntaxWarning}
+Base class for warnings about dubious syntax
+\end{excdesc}
+
+\begin{excdesc}{RuntimeWarning}
+Base class for warnings about dubious runtime behavior.
+\end{excdesc}