RFR (S) CR 8014966: Add the proper Javadoc to @Contended

[Aleksey Shipilev]

Hi,

It is time to add the proper Javadoc to @Contended:
  http://cr.openjdk.java.net/~shade/8014966/webrev.01/

Testing:
  - built jdk8-tl on Linux x86_64/release without a hitch.

Please review and sponsor!

Thanks,
-Aleksey.