this line is in 1.7.4
"cat_description" => format_text($cat_cache[$category_id]['cat_description'], 1),
when you after insert the line
"cat_description_short" => (strlen($cat_cache[$category_id]['cat_description']) > 120) ? substr($cat_cache[$category_id]['cat_description'], 0, 120)."..." : $cat_cache[$category_id]['cat_description'],
and replace in categories.html {cat_description} in {cat_description_short}
i have this code not in the online version, only offline!