make it easier to query multiple documents without a join

Building a query like the following:

```
SELECT * FROM [nt:unstructured] AS a
WHERE (
[phpcr:class] = 'AppBundle\Document\Article’ OR
[phpcr:class] = 'AppBundle\Document\Video’ OR
[phpcr:class] = 'AppBundle\Document\Crew’
)
AND
(
a.tags LIKE '%tag1%’ OR
a.tags LIKE '%tag2%’ OR
a.tags LIKE '%tag3%’
)
```

Maybe we could introduce a new method `$from->documents()` to be able to define that the `[phpcr:class]` filtering should be done on a set of documents?

wdyt @dantleech ?