Adding comments to Inspector via a component in Unity

CommentComponent.cs

#if UNITY_EDITOR
using UnityEditor;
#endif
using UnityEngine;

[AddComponentMenu("Comment")]
public class CommentComponent : MonoBehaviour
{
#if UNITY_EDITOR
    [SerializeField]
    private string comment;

    [CustomEditor(typeof(CommentComponent))]
    [CanEditMultipleObjects]
    public class CommentComponentEditor : Editor
    {
        private SerializedProperty commentProp;
        private GUIStyle commentStyle;
        private float inspectorWidth;

        protected void OnEnable()
        {
            commentProp = serializedObject.FindProperty("comment");
        }

        public override void OnInspectorGUI()
        {
            if (commentStyle == null)
            {
                commentStyle = new GUIStyle(EditorStyles.wordWrappedLabel);
                commentStyle.normal.textColor = commentStyle.focused.textColor = commentStyle.hover.textColor = Color.green;
            }

            serializedObject.Update();
            Rect rect = EditorGUILayout.GetControlRect(false, commentStyle.CalcHeight(new GUIContent(commentProp.stringValue), inspectorWidth));
            if (Event.current.type == EventType.Repaint)
                inspectorWidth = rect.width;

            EditorGUI.DrawRect(rect, Color.black); // Draw dark background
            EditorGUI.BeginProperty(rect, GUIContent.none, commentProp);

            EditorGUI.BeginChangeCheck();
            string newComment = EditorGUI.TextArea(rect, commentProp.stringValue, commentStyle);
            if (EditorGUI.EndChangeCheck())
                commentProp.stringValue = newComment;

            EditorGUI.EndProperty();
            serializedObject.ApplyModifiedProperties();
        }
    }
#endif
}

How To

Simply add this component to your objects. Note that it's unfortunately harder to edit the comment on Light editor skin due to the caret blending in with the background.

Cool!!!