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.