Added documentation for NotImplementedError. Fred (I'm sure) will

sanitize for your protection :-)

Doc/lib/libexcs.tex

@@ -241,6 +241,15 @@ Raised when an \keyword{assert} statement fails.
   more.)
 \end{excdesc}
 
+\begin{excdesc}{NotImplementedError}
+  \versionadded{1.5.2}
+  This exception is derived from \exception{RuntimeError}.  Its
+  purpose is to provide a standard exception to be used when modeling
+  abstract class methods.  Abstract base class methods should raise
+  this exception when they expect derived classes to override the
+  method.
+\end{excdesc}
+
 \begin{excdesc}{SyntaxError}
 % XXXJH xref to these functions?
   Raised when the parser encounters a syntax error.  This may occur in