Improve the definition of the "rare" builtin dictionary

For an end user who is not familiar with the inner details of the
script, the meaning of the old definition was ambiguous. It wasn't
clear whether using this dictionary flags the rare words as errors
(false positives) or on the contrary does not flag these rare words
(false negatives).

Co-authored-by: Peter Newman <peternewman@users.noreply.github.com>