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
}