Tweak Vahnatai PC traits and add Magery item ability

- Vahnatai no longer have a bonus to mage spells (that's now the Magery item ability)
- Vahnatai XP gain bonus changed
- Fix sliths being noted as resistant to poison (they aren't and never have been)

doc/editor/appendix/Items.html

@@ -155,10 +155,8 @@ otherwise, it's equal to one-half the Ability Strength.</dd>
 <dt>Ability 31 - Full Protection</dt>
 <dd>This partially blocks weapon and spell damage the wielder takes. It also protects from
 poison.</dd>
-<dt>Ability 32 - Melee Protection</dt>
-<dd>This item behaves as armour even if it isn't. It reduces damage by a die with the
-number of sides specified in Ability Strength. If the item is armour, this is on top of
-the normal damage reduction of the armour.</dd>
+<dt>Ability 32 - Magery</dt>
+<dd>This item makes your mage spells stronger.</dd>
 <dt>Ability 33 - Evasion</dt>
 <dd>This reduces the chance of enemies hitting you with melee attacks and non-magical
 missiles. The Ability Strength determines how much less likely they are to hit, as a