Add documentation for :negation
Add documentation for :negation

re: #118

undergroundwebdesigns/pg_search,rigoleto/pg_search,Casecommons/pg_search,paultyng/pg_search,undergroundwebdesigns/pg_search,mpivaa/pg_search,paultyng/pg_search,rigoleto/pg_search,mpivaa/pg_search,savef/pg_search,Casecommons/pg_search,savef/pg_search