Core/Vehicles: Give description to unknown seat flag

* There are 28 vehicles using for what is stated in the description  - it handles keeping specific emote states (animations) for passengers entering vehicles, I'm not 100% sure if is connected with the aura states that apply them. There are videos for every single known creature that is using it. 31583 - is clearest example since has only this flag as unknown on seat 0. Putting it here only to give direction to where researching should be made if someone dig into it more or can implement support for it.

src/server/game/DataStores/DBCEnums.h

@@ -406,7 +406,7 @@ enum VehicleSeatFlags
     VEHICLE_SEAT_FLAG_CAN_ATTACK                                       = 0x00004000, // Can attack, cast spells and use items from vehicle
     VEHICLE_SEAT_FLAG_SHOULD_USE_VEH_SEAT_EXIT_ANIM_ON_FORCED_EXIT     = 0x00008000,
     VEHICLE_SEAT_FLAG_UNK17                                            = 0x00010000,
-    VEHICLE_SEAT_FLAG_UNK18                                            = 0x00020000,
+    VEHICLE_SEAT_FLAG_UNK18                                            = 0x00020000, // Needs research and support (28 vehicles): Allow entering vehicles while keeping specific permanent(?) auras that impose visuals (states like beeing under freeze/stun mechanic, emote state animations).
     VEHICLE_SEAT_FLAG_HAS_VEH_EXIT_ANIM_VOLUNTARY_EXIT                 = 0x00040000,
     VEHICLE_SEAT_FLAG_HAS_VEH_EXIT_ANIM_FORCED_EXIT                    = 0x00080000,
     VEHICLE_SEAT_FLAG_UNK21                                            = 0x00100000,