WebSearch: restricted records in latest additions

* When a record belongs to both a public and a restricted collection,
  no longer display it in the list of latest additions of the
  public collection (in accordance to the outcome of an empty search query
  over the public collection).
  (closes #1315)

* Amend documentation of CFG_WEBSEARCH_VIEWRESTRCOLL_POLICY
  WRT default policy value.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>