JDK 9 RFR of 8161402: Provide a javadoc description for java.prefs module

[Brian Burkhalter]

Please review at your convenience.

Issue:	https://bugs.openjdk.java.net/browse/JDK-8161402
Patch:	[1]

Thanks,

Brian

[1] diff

--- a/src/java.prefs/share/classes/module-info.java
+++ b/src/java.prefs/share/classes/module-info.java
@@ -23,6 +23,10 @@
  * questions.
  */
 
+/**
+ * java.prefs defines and exports the preferences APIs of the Java SE platform.
+ */
+
 module java.prefs {
     requires java.xml;