[vector] more editorial changes, plus API tweaks

[John Rose]

As you might have noticed from my pushes to vector-unstable, I’ve been polishing the
Vector API draft, in preparation for our CSR.

Here is a fresh spin of the java doc:

http://cr.openjdk.java.net/~jrose/vectors/vector-unstable-doc

I think Kishor is working on a code example showing how to do masked loops correctly.

What else do we need before CSR?

— John