SF bug #719367, string exceptions are deprecated

Remove references to string based exceptions in the doc.
diff --git a/Doc/doc/doc.tex b/Doc/doc/doc.tex
index 22f9d13..1138554 100644
--- a/Doc/doc/doc.tex
+++ b/Doc/doc/doc.tex
@@ -608,8 +608,7 @@
     \end{envdesc}
 
     \begin{envdesc}{excdesc}{\p{name}}
-      Describe an exception.  This may be either a string exception or
-      a class exception.  In the case of class exceptions, the
+      Describe an exception.  In the case of class exceptions, the
       constructor parameters are not described; use \env{excclassdesc}
       to describe an exception class and its constructor.
     \end{envdesc}
@@ -1498,7 +1497,7 @@
 
     \begin{macrodesc}{exindex}{\p{exception}}
       Add a reference to an exception named \var{exception}.  The
-      exception may be either string- or class-based.
+      exception should be class-based.
     \end{macrodesc}
 
     \begin{macrodesc}{kwindex}{\p{keyword}}