RFR(XS): 6605915 jinfo -flag <flag name> functionality doesn't work with core files

[Staffan Larsen]

This is a small change to the usage message of jinfo to clarify that the -flag option is not available for core files.

The change is targeted for jdk9 and won’t be pushed until the jdk9 repos open up.

webrev: http://cr.openjdk.java.net/~sla/6605915/webrev.00/
bug: https://bugs.openjdk.java.net/browse/JDK-6605915

Thanks,
/Staffan