Consider not returning FALSE when saving a non-default entity revision

Problem/Motivation

In ContentEntityStorageBase::doSave():

    // @todo Consider returning a different value when saving a non-default
    //   entity revision. See https://www.drupal.org/node/2509360.
    $return = $entity->isDefaultRevision() ? SAVED_UPDATED : FALSE;

Proposed resolution

TBD

Remaining tasks

TBD

User interface changes

None

API changes

TBD

Data model changes

None