docs: def event_env_merge_info(app, env, _, other):
      """Called by Sphinx during phase 3 (resolving).
  
      * Combine child process' modified env with this one. Only changes should be new Imgur IDs since cache update is done
        in event_env_updated() after everything is merged and we're back to one process.
  
      :param sphinx.application.Sphinx app: Sphinx application object.
      :param sphinx.environment.BuildEnvironment env: Sphinx build environment.
      :param _: Not used.
      :param sphinx.environment.BuildEnvironment other: Sphinx build environment from child process.
      """
      other_album_cache = getattr(other, 'imgur_album_cache', None)
      other_image_cache = getattr(other, 'imgur_image_cache', None)
      if not other_album_cache and not other_image_cache:
          return
      album_cache = app.builder.env.imgur_album_cache
      image_cache = app.builder.env.imgur_image_cache
      assert env  # Linting.
  
      # Merge items.
      album_cache.update(other_album_cache)
      image_cache.update(other_image_cache)