RFR(xs) JDK-8163861 JNI NewString() and GetStringLength() documentation incorrect

[George Triantafillou]

Please review this very small fix to the spec for JDK-8163861:

JBS: https://bugs.openjdk.java.net/browse/JDK-8163861

webrev: http://cr.openjdk.java.net/~gtriantafill/8163861-webrev/webrev 
<http://cr.openjdk.java.net/%7Egtriantafill/8163861-webrev/webrev>

The change provides a more precise definition for NewString() and 
GetStringLength().  Thanks.

-George