MigrationInterface::mergeProcessOfProperty() doesn't document who wins

Problem/Motivation

The docs say:

   * Merge the process pipeline configuration for a single property.
   *
   * @param string $property
   *   The property of which to merge the passed in process pipeline
   *   configuration.
   * @param array $process_of_property
   *   The process pipeline configuration to be merged with the existing process
   *   pipeline configuration.
   *
   * @return $this
   *   The migration entity.

This leaves a lot unsaid. If the same property is in the migration configuration and the incoming $process_of_property configuration, which one wins? Is the merge deep for arrays?

Steps to reproduce

Proposed resolution

Remaining tasks

Investigate what the implementation does and expand the documentation.

User interface changes

API changes

Data model changes

Release notes snippet