Add warning that last() and previous() don't work for hashtable

databases (opened using hashopen()), as noted by Skip Montanaro on
comp.lang.python.

Doc/lib/libbsddb.tex

@@ -104,13 +104,15 @@ keys in the file is unspecified.
 \end{methoddesc}
 
 \begin{methoddesc}{previous}{}
-Set the cursor to the first item in the DB file and return it.  The order of 
-keys in the file is unspecified.
+Set the cursor to the first item in the DB file and return it.  The
+order of keys in the file is unspecified.  This is not supported on
+hashtable databases (those opened with \function{hashopen()}).
 \end{methoddesc}
 
 \begin{methoddesc}{last}{}
-Set the cursor to the last item in the DB file and return it.  The order of 
-keys in the file is unspecified.
+Set the cursor to the last item in the DB file and return it.  The
+order of keys in the file is unspecified.  This is not supported on
+hashtable databases (those opened with \function{hashopen()}).
 \end{methoddesc}
 
 \begin{methoddesc}{sync}{}