[master] clarification about cache-enable

discussed with vorner via jabber.

doc/guide/bind10-guide.xml

@@ -1765,8 +1765,10 @@ can use various data source backends.
         answering up. The first option is <varname>cache-enable</varname>,
         a boolean value turning the cache on and off (off is the default).
         The second one, <varname>cache-zones</varname>, is a list of zone
-        origins to load into in-memory. Remember that zones in the data source
-        not listed here will not be loaded and will not be available at all.
+        origins to load into in-memory. Once the cache is enabled,
+        the zones in the data source not listed in
+        <varname>cache-zones</varname> will not be loaded and will
+        not be available at all.
       </para>
 
       <section id='datasource-types'>