The messages:
# Running javadoc for images/docs/jre/api/management/rm/index.html
javadoc: warning - Error reading file: c:\workspaces\hg-dev9-jake\jdk9\build\windows-x86_64-normal-server-release\images\docs\api\package-list
javadoc: warning - Error reading file: c:\workspaces\hg-dev9-jake\jdk9\build\windows-x86_64-normal-server-release\images\docs\api\package-list

Note: the messages reflects jake, but this is also reproducible in the jdk9 builds.

The problem: I think the api docs are all spawned together to build the doc bundles, however some of the docs link coredocs, and the smaller/faster docs bundles which complete sooner, will not be able to access the coredocs files.

The solution: I think coredocs needs to be forked first, and the other doc bundle jobs must wait for the coredocs to complete first.

Another data point supporting the theory :)
make JOBS=1 docs, works fine.