WebSearch: add refersto:/a b c/ example to guide
WebSearch: add refersto:/a b c/ example to guide

* Enrich refersto/citedby section of the Search Guide by a more complex
  example of a phrase and regexp quoting in the search operand.
  (refersto:/author:"Klebanov, I" title:O(N)/)

tiborsimko/invenio,inveniosoftware/invenio,inveniosoftware/invenio,tiborsimko/invenio