[gtk+] Document CSS win32 size support

[Matthias Clasen <matthiasc src gnome org>]

commit d3f713529ab2c1fcbf1a35de0a3a38cf9fb49399
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Feb 26 00:13:34 2016 -0500

    Document CSS win32 size support

 docs/reference/gtk/css-properties.xml |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/gtk/css-properties.xml b/docs/reference/gtk/css-properties.xml
index dc87af5..76a1482 100644
--- a/docs/reference/gtk/css-properties.xml
+++ b/docs/reference/gtk/css-properties.xml
@@ -70,6 +70,19 @@ We use <literallayout> for syntax productions, and each line is put in a <code>
       not quite the same as the CSS definition of rem.
     </para>
 
+    <para>
+      Whereever a number is allowed, GTK+ also accepts a Windows-specific
+      theme size:
+    </para>
+
+<literallayout>
+<code>〈win32 theme size〉 = 〈win32 size〉| 〈win32 part size〉</code>
+<code>〈win32 size〉 = -gtk-win32-size ( 〈theme name〉, 〈metric id〉 )</code>
+<code>〈win32 part size〉 = [ -gtk-win32-part-width | -gtk-win32-part-height |</code>
+<code>                         -gtk-win32-part-border-top | -gtk-win32-part-border-right |</code>
+<code>                         -gtk-win32-part-border-bottom | -gtk-win32-part-border-left ] ( 〈theme name〉 
, 〈integer〉 , 〈integer〉 )</code>
+</literallayout>
+
 <literallayout><code>〈calc expression〉 = calc( 〈calc sum〉 )</code>
 <code>〈calc sum〉 = 〈calc product〉 [ [ + | - ] 〈calc product〉 ]*</code>
 <code>〈calc product〉 = 〈calc value〉 [ * 〈calc value〉 | / 〈number〉 ]*</code>