How do I attach Javadocs for a Gradle dependency in Eclipse?

My Gradle project is pulling in some jar dependencies from file like this:

dependencies {
    compile fileTree(dir: 'lib', include: '*.jar')
}

I have downloaded the Javadocs for one of the dependencies as a zip file, how do I add the Javadocs in Eclipse?

When I right click on the Gradle dependency and try to add Javadocs I see this:

The current class path entry belongs to container 'Gradle Dependencies (persisted)' which does not allow user modifications to Javadoc locations on its entries.