RFR JDK-8046219: (str spec) String(byte[], int, int, Charset) should be clearer when IndexOutOfBoundsException is thrown

[Xueming Shen]

Hi

Please help review this doc only change, in which the rewording precisely 
specifies
when the IndexOutOfBoundsException should be thrown, while be compatible with
the long standing behavior.

issue: https://bugs.openjdk.java.net/browse/JDK-8046219
webrev: http://cr.openjdk.java.net/~sherman/8046219

Thanks!
-Sherman