Clarify implementation note in Clock.java to match implementation changes made by JDK-8068730

In JDK 9, the implementation of Clock has been improved to be better
than millisecond in most cases. However, the Javadoc of the "Implementation Note" is now wrong. It says:

"The clock implementation provided here is based on
System.currentTimeMillis(). That method provides little to no
guarantee about the accuracy of the clock. Applications requiring a
more accurate clock must implement this abstract class themselves
using a different external clock, such as an NTP server."

This needs to be updated to match the changes to the implementation.
Suggested replacement text:

"The clock implementation provided here is based on the same
underlying clock as System.currentTimeMillis(), but may have a
precision finer than milliseconds if available. However, little to no
guarantee is provided about the accuracy of the underlying clock.
Applications requiring a more accurate clock must implement this
abstract class themselves using a different external clock, such
 as an NTP server."