Explain query results on activity pages

## Feature Request Form
### Problem you are trying to address with this feature
Because our API returns only 200 annotations per query, the number of annotations shown for each document (and also in uncollapsed view, the specific annotations on a given document) will often be incomplete.

Thus, users will naturally think they are looking at all of the annotations of a given document, and will fail to realise that they're not.

### Your solution
It would help clarify what users are seeing if, somewhere unobtrusive, like perhaps at the bottom of the page we specified exactly what the results are. @seanh and I discussed a sentence saying something like:

"This page shows 200 annotations across 5 documents between now and Dec 4, 16:32pm"

For the second page, it might say:

"This page shows 200 annotations across 8 documents between Dec 2, 6:32am and Dec 4, 16:32pm"

That way the users will at least have a hint as to why the number of annotations doesn't reflect what they remember seeing on the actual document itself.

Also, within the uncollapsed document view, we might provide some similar text, such as:
"7 users created 10 matching annotations on this document between Dec 4, 16:32pm and Dec 6, 8:12am. [See all]" (See all >> Same link as the view annotations in context link).

A challenge with this last recommendation would be where to display it for documents with only one small card, since there might not be sufficient vertical space to the side.