Deprecate SEARCH_QUERY() function in release notes

This may sound like a strange request, but
Prior to the official search() functions, query / FTS integration had been implemented via a search_query() function, which has never been documented, but nonetheless is still available.

search_query() had been externalized in aDZone blog, back in the 6.5 days, and we have had defects logged against it: in short, although it's never been documented, it is visible, and since we are considering retiring it in cheshire_cat.next, it seems like a good idea deprecating it in the release notes in cheshire_cat.