Add to the applet caching chapter in the plugin document.

Add to known issues or notes section:

If you get the "java.io.IOException: Caching not supported for ..."
exception that means the Java Plug-in was unable to get the expiration date and
last modification date for a given JAR file from the web server. If the Java
Plug-in can not get this information, there is no point in caching since the JAR
file will be downloaded every time it is used. Even with this exception, the
applet should work fine. The exception is just informative.