RFR 8177291: [doc] weak algorithms and crypto policy in JGSS docs

[Weijun Wang]

This is not exactly a code review, I'd like you to review my suggested 
changes on the JGSS guides in

  https://bugs.openjdk.java.net/browse/JDK-8177291
If everything is OK, I can pass it to the doc writer.

Thanks
Max