Comment.cs

// This adds a simple text area for you to add comments
// to your GameObjects in the Unity inspector.
// Comments are saved with your scene, just like any other
// component on a GameObject.

using UnityEngine;
#if UNITY_EDITOR
using UnityEditor;
#endif

public class Comment : MonoBehaviour
{   
    public string comment = "";
}

#if UNITY_EDITOR

[CustomEditor(typeof(Comment))]
public class CommentEditor : Editor 
{
    public override void OnInspectorGUI()
    {
        Comment _target = (Comment)target;
        EditorGUI.BeginChangeCheck();

        string newValue = GUILayout.TextArea(_target.comment);

        if (EditorGUI.EndChangeCheck())
        {
            Undo.RecordObject(target, "Changed Comment");
            _target.comment = newValue;
        }
    }
}

#endif