Psyllium in Cereal: Unknown Territory
LEAD: SINCE April, the word psyllium has probably appeared in print more times than it had in all the five centuries after Gutenberg. General Mills and Procter & Gamble are responsible for its sudden notoriety: General Mills because it introduced a cereal called Benefit that contains psyllium, and Procter & Gamble because it wants the Food and Drug Administration to do something about health claims made for the General