Document filter message for no results

www.agileplannerapp.com/boards/173808/cards/8672
addresses: https://govuk.zendesk.com/tickets/869065

In its current version the document filter shows
a loader image when the search is being performed
and doesn't provide any feedback if results are
not found.

Editors can get confused if they're searching for
a publication to link to a Statistics Announcement
and no results are returned. It may seem as if the
filter is not functioning.

I've added a suitable message which reminds editors
that the lookup is scoped by publication type which
is same as the type of the statistics announcement,
and, publication state being not published.
Merge pull request #1847 from alphagov/add-no-results-message-to-document-filter

Document filter message for no results