javadoc --release could auto-link to JDK docs

Just as javac --release implies a virtual bootclasspath or modular equivalent, so too could javadoc --release imply an implicit --link-offline to the JDK docs.

Maven tries to do this, but it seems like this should be more core responsibility. It would mean caching (in the repo) the package-list or element-list for the supported releases.

We'd maybe want to be able to disable or retarget the links.