You can use index rollup in three ways:
3. Specify the index rollup job as an ISM action for complete index management. This allows you to roll up an index after a certain event such as a rollover, index age reaching a certain point, index becoming read-only, and so on. You can also have rollover and index rollup jobs running in sequence, where the rollover first moves the current index to a warm node and then the index rollup job creates a new index with the minimized data on the hot node.
It looks like documentation is not updated, will attempt to update soon.
The feature is released and available since 1.13.0 I believe. Here is a sample ISM policy to create the rollup.
PS: At the moment only single run rollup jobs can be created through this path, the source index for the rollup job is inferred from the index the policy is attached to.