[gtk+] Add GtkIconSize back to the docs

[Matthias Clasen <matthiasc src gnome org>]

commit d04f66eb524294e7de55e87a0104c457033c1dc5
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Dec 26 08:39:29 2017 -0500

    Add GtkIconSize back to the docs
    
    And document how it works now.

 docs/reference/gtk/css-properties.xml |    2 +-
 docs/reference/gtk/gtk4-sections.txt  |    1 +
 gtk/gtkenums.h                        |    4 ++++
 3 files changed, 6 insertions(+), 1 deletions(-)
---
diff --git a/docs/reference/gtk/css-properties.xml b/docs/reference/gtk/css-properties.xml
index 81be2ef..42837b0 100644
--- a/docs/reference/gtk/css-properties.xml
+++ b/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>
diff --git a/docs/reference/gtk/gtk4-sections.txt b/docs/reference/gtk/gtk4-sections.txt
index bea54de..9a25b31 100644
--- a/docs/reference/gtk/gtk4-sections.txt
+++ b/docs/reference/gtk/gtk4-sections.txt
@@ -5300,6 +5300,7 @@ GtkShadowType
 GtkStateFlags
 GtkToolbarStyle
 GtkSortType
+GtkIconSize
 
 </SECTION>
 
diff --git a/gtk/gtkenums.h b/gtk/gtkenums.h
index a75429b..81c20f1 100644
--- a/gtk/gtkenums.h
+++ b/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
 {