docs: Clarify gtk_render_icon()'s behaviour

A GdkPixbuf has no scaling factor, so drawing directly from it can only
using a scale of 1, to avoid blurry, fuzzy icons.

You should be using gtk_render_icon_surface() anyway.

gtk/gtkrender.c

@@ -1205,6 +1205,13 @@ gtk_render_icon_pixbuf (GtkStyleContext     *context,
  *
  * Renders the icon in @pixbuf at the specified @x and @y coordinates.
  *
+ * This function will render the icon in @pixbuf at exactly its size,
+ * regardless of scaling factors, which may not be appropriate when
+ * drawing on displays with high pixel densities.
+ *
+ * You probably want to use gtk_render_icon_surface() instead, if you
+ * already have a Cairo surface.
+ *
  * Since: 3.2
  **/
 void