[vector] RFR 8222752: Javadoc changes for Vector api

[Kharbas, Kishor]

Hi all,
I have a patch refining Javadoc for vector api as described in https://bugs.openjdk.java.net/browse/JDK-8222752
The refinements were suggested by John Rose, and reviewed by him.

Webrev - http://cr.openjdk.java.net/~kkharbas/vector-api/javadoc/webrev-javadoc.03/
Generated Javadoc - http://cr.openjdk.java.net/~kkharbas/vector-api/javadoc/VectorApi-JavaDoc.03/jdk.incubator.vector/jdk/incubator/vector/package-summary.html

Thanks,
Kishor