Fix a minor typo and mark an exception name that was missed.

Doc/lib/libwarnings.tex

@@ -55,7 +55,7 @@ currently defined:
 \begin{tableii}{l|l}{exception}{Class}{Description}
 
 \lineii{Warning}{This is the base class of all warning category
-classes.  It itself a subclass of Exception.}
+classes.  It is a subclass of \exception{Exception}.}
 
 \lineii{UserWarning}{The default category for \function{warn()}.}