To the best of my knowledge the only way you would achieve that within RoboHelp is to make the help Section 508 compliant. That means you cannot have a skin so it's probably not what you will want.
The alternative is a very extensive index but that is not quite the same thing.
A third party tool such as Zoomsearch (described on my site) but that's more work.
Nowadays most people are used to ranked searching because of Google. Perhaps you could use that argument?
See www.grainge.org for RoboHelp and Authoring information

@petergrainge
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