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)/)

* Cherry-picked from ef648f1f91e78153349f9b6b1d8a0755fc3b1593.

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