Here is a custom control for editing enum fields where custom descriptions can be specified by decorating values with System.ComponentModel.DescriptionAttribute:

 using UnityEngine;
 using UnityEditor;

 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Linq;

 public static class CustomGUI {
     // Cache of enum values.
     private static Dictionary<Type, int[]> s_EnumValueCache = new Dictionary<Type, int[]>();
     // Cache of enum descriptions/nicified values.
     private static Dictionary<Type, string[]> s_EnumDescriptionCache = new Dictionary<Type, string[]>();

     public static int[] GetEnumValues<T>() {
         if (!s_EnumValueCache.ContainsKey(typeof(T)))
             s_EnumValueCache[typeof(T)] = Enum.GetValues(typeof(T)).Cast<int>().ToArray();
         return s_EnumValueCache[typeof(T)];
     }

     public static string[] GetEnumDescriptions<T>() {
         if (!s_EnumDescriptionCache.ContainsKey(typeof(T))) {
             var list = new List<string>();
             foreach (var value in Enum.GetValues(typeof(T))) {
                 var attribute = typeof(T).GetMember(value.ToString())[0].GetCustomAttributes(typeof(DescriptionAttribute), false).FirstOrDefault() as DescriptionAttribute;
                 list.Add( attribute != null ? attribute.Description : ObjectNames.NicifyVariableName(value.ToString()) );
             }
             s_EnumDescriptionCache[typeof(T)] = list.ToArray();
         }
         return s_EnumDescriptionCache[typeof(T)];
     }

     public static int EnumPopup<T>(Rect position, int value) {
         var values = GetEnumValues<T>();
         int selectedIndex = EditorGUI.Popup(position, Array.IndexOf(values, value), GetEnumDescriptions<T>());
         return (selectedIndex >= 0 && selectedIndex < values.Length) ? values[selectedIndex] : value;
     }
 }

Usage Example:

An enum can then be defined as follows:

 using System.ComponentModel;

 public enum MyEnum {
     [Description("More detailed description for ABC")]
     ABC,
     [Description("More detailed description for DEF")]
     DEF,
     LetUnityNicifyName,
 }

Which can be presented within a custom GUI:

 private MyEnum _selectedEnumValue = MyEnum.ABC;

 private void OnGUI() {
     Rect position = GUILayoutUtility.GetRect(GUIContent.none, EditorStyles.popup);
     _selectedEnumValue = (MyEnum)CustomGUI.EnumPopup<MyEnum>(position, (int)_selectedEnumValue);
 }