[FIX] unified: Rename feature_search pref to "Unified Search Index" and add a remarksbox note to explain a bit more about the other things Unified Index does (addresses wish4666 - could still be improved? Thanks marclaporte)

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/12.x@48276 b456876b-0849-0410-b77d-98878d47e9d5