return evaluation of search result precision based on search string

[jeanetteclark]

Once free text search is enabled, we should figure out how to let users know the precision of the match that is expected based on the search string. We'd need to think of the exact language to return, but it should cover at least these 3 cases:

search term	precision estimate
resolvable identifier	dataset definitely cited by journal
dataset title	dataset possibly referenced by journal
generic search string	a list of stuff that may or may not contain references to datasets