Document the return value of getEffectMultiplier.

apps/openmw/mwmechanics/spellcasting.hpp

@@ -51,6 +51,10 @@ namespace MWMechanics
     float getEffectResistance (short effectId, const MWWorld::Ptr& actor, const MWWorld::Ptr& caster,
                                const ESM::Spell* spell = NULL, const MagicEffects* effects = NULL);
 
+    /// Get an effect multiplier for applying an effect cast by the given actor in the given spell (optional).
+    /// @return effect multiplier from 0 to 2.  (100% net resistance to 100% net weakness)
+    /// @param effects Override the actor's current magicEffects. Useful if there are effects currently
+    ///                being applied (but not applied yet) that should also be considered.
     float getEffectMultiplier(short effectId, const MWWorld::Ptr& actor, const MWWorld::Ptr& caster,
                               const ESM::Spell* spell = NULL, const MagicEffects* effects = NULL);