[gtk+] Document -gtk-icon-size

[Matthias Clasen <matthiasc src gnome org>]

commit f16fb1373e1d494c7a075c4bcb5574ac95cf652c
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Nov 15 13:14:36 2017 -0500

    Document -gtk-icon-size
    
    Add this property to the list of CSS properties.

 docs/reference/gtk/css-properties.xml |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)
---
diff --git a/docs/reference/gtk/css-properties.xml b/docs/reference/gtk/css-properties.xml
index b7cb972..f8e4c8e 100644
--- a/docs/reference/gtk/css-properties.xml
+++ b/docs/reference/gtk/css-properties.xml
@@ -470,8 +470,17 @@ We use <literallayout> for syntax productions, and each line is put in a <code>
               <entry><code>none</code></entry>
               <entry>✓</entry>
               <entry></entry>
-              <entry></entry>
               <entry><ulink url="https://drafts.fxtf.org/filters/#FilterProperty";>CSS3</ulink></entry>
+              <entry></entry>
+            </row>
+            <row>
+              <entry>‑gtk‑icon‑size</entry>
+              <entry><code>〈length〉</code></entry>
+              <entry><code>none</code></entry>
+              <entry>✓</entry>
+              <entry></entry>
+              <entry></entry>
+              <entry>Determines the size at which icons are displayed</entry>
             </row>
           </tbody>
         </tgroup>