WP = Weapon Proficiency, as in: You are knowledgeable about and can use... Any Bludgeoning weapon and any Cloth, Leather, and Chain armor. Greyhawk is an old world, goverened mostly by 1st edition rules (the only edition i ever played it in too). Clerics of Pelor were Plate restricted because they werent meant to be battle clerics from a religious standpoint. In exchange for the armor sacrifice, they got better healing appoximately equivalent of the Augment Healing feat. And as for weapons, before 3E, clerics couldn't use non blunt weapons because their ethos, or system of beliefs, basicly reviled the shedding of blood. Sure, blunt weapons cause bleeding too, but to a far lesser extent than a sword or a pike.
Hope this helps.
WarShrike