RFR(XS): JDK-8145247 incorrect comment in SystemDictionary::load_shared_class

[Ioi Lam]

Hi,

Please review a very small fix:

     https://bugs.openjdk.java.net/browse/JDK-8145247
http://cr.openjdk.java.net/~iklam/jdk9/8145247-system-dict-comments/

The comments are updated to reflect the latest behavior. Also, added 2
asserts to validate that the behavior is the same as the new comment
says.

Thanks
- Ioi