Review request for JDK-8139590: Improve Dynalink JavaDoc

[Attila Szegedi]

Please review JDK-8139590 "Improve Dynalink JavaDoc" at <http://cr.openjdk.java.net/~attila/8139590/webrev.jdk9> for <https://bugs.openjdk.java.net/browse/JDK-8139590>

Notes:
- webrev also shows some previous issues in the history of some of the files, but the changes for those issue are not in the diffs.
- In addition to JavaDoc, there are some minor additional fixes:
  - GuardedInvocation.assertType was removed; it has shown up in JavaDoc, but it was not used except for one place in TypeConverterFactory, and even there it wasn’t needed
  - GuardedInvocation had an accidental dependency on jdk.nashorn.internal.* class; that was eliminated.
  - AbstractCallSiteDescriptor.lookupsEqual and lookupHashCode now handle nulls
  - CompositeGuardingDynamicLinker and TypeBasedCompositeGuardingDynamicLinker detect if any of its composites are null and throw a NPE

Thanks,
  Attila.