JDK 9 doc-api-only RFR of 7086489: File.lastModified should accuracy as well as resolution

[Brian Burkhalter]

Please review at your convenience.

Issue:	https://bugs.openjdk.java.net/browse/JDK-7086489
Patch:	[1]

Thanks,

Brian

[1] Hg mq diff

--- a/src/java.base/share/classes/java/io/File.java
+++ b/src/java.base/share/classes/java/io/File.java
@@ -923,6 +923,12 @@
      * java.nio.file.Files#readAttributes(Path,Class,LinkOption[])
      * Files.readAttributes} method may be used.
      *
+     * @implSpec While the unit of the returned value is milliseconds, this
+     * does not imply that its accuracy is to the millisecond as some platforms
+     * may truncate the last-modified time to a less accurate value. For
+     * example if the value were truncated to seconds, then the last three
+     * digits of the returned value would all be zero.
+     *
      * @return  A <code>long</code> value representing the time the file was
      *          last modified, measured in milliseconds since the epoch
      *          (00:00:00 GMT, January 1, 1970), or <code>0L</code> if the