Questinfo correction (#5523)

* Corrected questinfo bubble not always being displayed
* Docs updated
* Changed qi_display to std::vector

Co-authored-by: Lemongrass3110 <lemongrass@kstp.at>

Thanks to @lighta @aleos89 @Lemongrass3110 !

src/map/npc.cpp

@@ -2483,6 +2483,8 @@ int npc_unload(struct npc_data* nd, bool single) {
 		}
 	}
 
+	nd->qi_data.clear();
+
 	script_stop_sleeptimers(nd->bl.id);
 	aFree(nd);