RFR: JDK-8172312 Update docs target and image for new combined docs

[mark.reinhold at oracle.com]

2017/3/30 6:05:43 -0700, magnus.ihse.bursie at oracle.com:
> As a part of JEP 299, we should build the Javadoc as a single combined 
> output, instead of a dozen or so individual javadoc bundles. This bug 
> fixes this. The selection on what to include is now based on modules 
> instead of packages.

I'm worried that perhaps we've unified a bit too much here.

This patch does build all the Javadoc together, as advertised, but that
places the `jdk.*` modules together with the `java.*` modules under the
heading "Java Platform, Standard Edition 9 API Specification", and of
course the `jdk.*` modules aren't part of that spec.

JDK 9 is the "Reference Implementation" (in JCP terms) of Java SE 9, so
it's important to keep a clear distinction between the SE-standard parts
of the spec and those that are JDK-specific.

Would it make sense instead to have `docs/api` be just for the SE (i.e.,
`java.*`) modules, and then place the Javadoc for the `jdk.*` modules
under `docs/api/jdk`?  There could be a helpful link from the SE API
title page to the JDK API title page, but it should make it clear that
the JDK APIs aren't part of the SE spec.

- Mark