Improve messaging re document limit for term frequency

If the term frequency graph has to search through many results, it only searches a sample. If that is the case, the interface shows a message through the user that I quickly put together, but the user friendliness could be improved here.

A comment from Pasi about this:

> In case we cannot get total data with enquiries with a more common term I think that the interface should clearly state where the limit of hits that can be processed fully is in each corpus.