Add GtkIconSize back to the docs

And document how it works now.

docs/reference/gtk/css-properties.xml

@@ -569,7 +569,7 @@ We use <literallayout> for syntax productions, and each line is put in a <code>
               <entry>✓</entry>
               <entry></entry>
               <entry></entry>
-              <entry>Determines the size at which icons are displayed</entry>
+              <entry>Determines the size at which icons are displayed. See GtkIconSize</entry>
             </row>
           </tbody>
         </tgroup>

docs/reference/gtk/gtk4-sections.txt

@@ -5300,6 +5300,7 @@ GtkShadowType
 GtkStateFlags
 GtkToolbarStyle
 GtkSortType
+GtkIconSize
 
 </SECTION>
 

gtk/gtkenums.h

@@ -183,6 +183,10 @@ typedef enum
  *
  * Icon sizes default to being inherited. Where they cannot be
  * inherited, text size is the default.
+ *
+ * All widgets which use GtkIconSize set the normal-icons or large-icons
+ * style classes correspondingly, and let themes determine the actual size
+ * to be used with the -gtk-icon-size CSS property.
  */
 typedef enum
 {