The comments for the Aggregator storage classes define the classes controllers, when they are storage handlers

The comment before the ItemStorage class is the following one.

/**
 * Controller class for aggregators items.
 *
 * This extends the Drupal\Core\Entity\Sql\SqlContentEntityStorage class, adding
 * required special handling for feed item entities.
 */

That class isn't a controller class, but a storage handler class. That comment should be changed to the following one, which is similar to the comment used in NodeStorage.php.

/**
 * Defines the storage handler class for feed item entities.
 *
 * This extends the base storage class, adding required special handling for
 * feed item entities.
 */

I would replace aggregators items since it's not correct. (The appositive that precedes the referenced noun phrase is a singular noun phrase.) I would also avoid to make explicit which is the base class, as in this context isn't necessary. (Furthermore, the base class is already documented in the code, which is visible in the documentation page.)

The same happens in FeedStorage.php.

/**
 * Controller class for aggregator's feeds.
 *
 * This extends the Drupal\Core\Entity\Sql\SqlContentEntityStorage class, adding
 * required special handling for feed entities.
 */

In this case, I would change it to the following comment.

/**
 * Defines the storage handler class for feed entities.
 *
 * This extends the base storage class, adding required special handling for
 * feed entities.
 */