RFR JDK-8250219: Proxy::newProxyInstance spec should specify the behavior if a given proxy interface is hidden

[Mandy Chung]

CSR: https://bugs.openjdk.java.net/browse/JDK-8250224

Webrev: http://cr.openjdk.java.net/~mchung/jdk16/webrevs/8250219/webrev.00/

This patch updates the `|Proxy::newProxyInstance` |spec to explicitly 
list that the given interfaces must be non-hidden interfaces.|  
Proxy::newProxyInstance| currently throws IAE if a given proxy interface 
cannot be found by the given class loader. This covers for the case of a 
hidden proxy interface since it is not discoverable by any class loader. 
However the spec should clearly document the behavior about hidden 
interfaces and the exception message indicates if the interface is 
hidden clearly.

thanks
Mandy