[PP-1] bundle_key configuration for entity_lookup process plugin isn't necessary

Yes this can create some nasty bugs.

We ended up associating terms from the wrong vocabulary because the documentation isn't clear that you both bundle AND bundle_key are required, it just says:

 * - bundle_key: (optional) The name of the bundle field on the entity type
 *   being queried.
 * - bundle: (optional) The value to query for the bundle - can be a string or
 *   an array.

This implies that both are optional and makes no mention that `bundle` is useless without `bundle_key`.

I also saw this, where I was using a most basic set up of only entity_generate (same configuration as entity_lookup) based on the doc page Contrib process plugin: entity_generate → (since updated), but had to supply a lot more configuration to make it work, as in comment #7. See #2787219-30: Entity lookup plugin not able to guess config for entity types without bundle → .