[libhandy] action-row: Document an unfixable ABI issue

[Alexander Mikhaylenko <alexm src gnome org>]

commit fb54ff0c14c71e83dd5abade052ef3ae7c5c3d9e
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Thu May 20 09:21:48 2021 +0200

    action-row: Document an unfixable ABI issue

 src/hdy-action-row.h      | 3 +++
 src/hdy-preferences-row.h | 3 +++
 2 files changed, 6 insertions(+)
---
diff --git a/src/hdy-action-row.h b/src/hdy-action-row.h
index 5b7121a3..b2c9074b 100644
--- a/src/hdy-action-row.h
+++ b/src/hdy-action-row.h
@@ -28,6 +28,9 @@ G_DECLARE_DERIVABLE_TYPE (HdyActionRow, hdy_action_row, HDY, ACTION_ROW, HdyPref
  */
 struct _HdyActionRowClass
 {
+  /* FIXME This should be HdyPreferencesRowClass but we can't change it without
+   * breaking the ABI.
+   */
   GtkListBoxRowClass parent_class;
 
   void (*activate) (HdyActionRow *self);
diff --git a/src/hdy-preferences-row.h b/src/hdy-preferences-row.h
index f5e926b3..c8f720a2 100644
--- a/src/hdy-preferences-row.h
+++ b/src/hdy-preferences-row.h
@@ -30,6 +30,9 @@ struct _HdyPreferencesRowClass
   GtkListBoxRowClass parent_class;
 
   /*< private >*/
+  /* FIXME We can't use the padding because of an error in HdyActionRowClass
+   * that we can't fix without breaking the ABI.
+   */
   gpointer padding[4];
 };