RFR (XS): 8183273: Clarify Instrumentation interface should not be implemented outside java.instrument module

[serguei.spit...@oracle.com]

Please, review a fix for java.lang.instrument.Instrumentation spec update:   https://bugs.openjdk.java.net/browse/JDK-8183273 Related CSR:   https://bugs.openjdk.java.net/browse/JDK-8223740 Webrev:   http://cr.openjdk.java.net/~sspitsyn/webrevs/2019/8183273-Instr-note.1/ Summary:   A clarification note is added to the https://bugs.openjdk.java.net/browse/JDK-8223740. Thanks, Serguei