Add documentation for the StopIteration exception.

Doc/lib/libexcs.tex

@@ -237,6 +237,14 @@ Raised when an \keyword{assert} statement fails.
   more.)
 \end{excdesc}
 
+\begin{excdesc}{StopIteration}
+  Raised by an iterator's \method{next()} method to signal that there
+  are no further values.
+  This is derived from \exception{Exception} rather than
+  \exception{StandardError}, since this is not considered an error in
+  its normal application.
+\end{excdesc}
+
 \begin{excdesc}{SyntaxError}
 % XXXJH xref to these functions?
   Raised when the parser encounters a syntax error.  This may occur in