Use the new module synopsis syntax, even though we don't need a synopsis table

for this chapter.  This protects against backend changes for \declaremodule.

Doc/lib/libexcs.tex

@@ -1,6 +1,7 @@
 \section{Built-in Exceptions}
-\label{module-exceptions}
-\stmodindex{exceptions}
+\declaremodule{standard}{exceptions}
+
+\modulesynopsis{Standard exceptions classes.}
 
 
 Exceptions can be class objects or string objects.  While