JDK 9 RFR of 8166253: (ch) FileLock object can get GC'd and result in unexpected release of file lock

[Brian Burkhalter]

Please review at your convenience.

Issue:	https://bugs.openjdk.java.net/browse/JDK-8166253
Patch:	http://cr.openjdk.java.net/~bpb/8166253/webrev.00/

Add an implementation note indicating that the lock could be released if the caller does not retain the returned object as a strongly reachable variable. If this looks reasonable a CCC request will be filed and the issue labels updated as needed.

Thanks,

Brian