fix(Core/Unit): Add mana drain effect to Black Bow of the Betrayer. (#20155)

* Init.

Co-authored by: stfxpi (stfx)
Original CMaNGOS commit: c750533b5e

* Add/adjust comments.

src/server/game/Entities/Unit/Unit.cpp

@@ -8987,9 +8987,9 @@ bool Unit::HandleProcTriggerSpell(Unit* victim, uint32 damage, AuraEffect* trigg
                             if (GetStat(STAT_AGILITY)  > stat) { trigger_spell_id = 67772;                               }
                             break;
                         }
-                    // Mana Drain Trigger
-                    case 27522:
-                    case 40336:
+                    case 27522: // Mana Drain Trigger
+                    case 40336: // Mana Drain Trigger
+                    case 46939: // Black Bow of the Betrayer
                         {
                             // On successful melee or ranged attack gain $29471s1 mana and if possible drain $27526s1 mana from the target.
                             if (IsAlive())