[gtk+] Document -gtk-icon-filter

[Matthias Clasen <matthiasc src gnome org>]

commit 8a83362fb88c69354de2e1a34a9fe2b01b712ba8
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Jan 10 21:12:38 2017 -0500

    Document -gtk-icon-filter
    
    Not much detail here yet, but the syntax is documented.

 docs/reference/gtk/css-properties.xml |   12 ++++++++++--
 1 files changed, 10 insertions(+), 2 deletions(-)
---
diff --git a/docs/reference/gtk/css-properties.xml b/docs/reference/gtk/css-properties.xml
index e71f49e..e992f11 100644
--- a/docs/reference/gtk/css-properties.xml
+++ b/docs/reference/gtk/css-properties.xml
@@ -444,8 +444,8 @@ We use <literallayout> for syntax productions, and each line is put in a <code>
               <entry></entry>
             </row>
             <row>
-              <entry>‑gtk‑icon‑effect</entry>
-              <entry><code>none | highlight | dim</code></entry>
+              <entry>‑gtk‑icon‑filter</entry>
+              <entry><code>none | 〈filter〉</code></entry>
               <entry><code>none</code></entry>
               <entry>✓</entry>
               <entry></entry>
@@ -466,6 +466,14 @@ We use <literallayout> for syntax productions, and each line is put in a <code>
 <code>                rotateX( 〈angle〉 ) | rotateY( 〈angle〉 ) | rotateZ( 〈angle〉 ) |</code>
 <code>                skew( 〈angle〉 [ , 〈angle〉 ]? ) | skewX( 〈angle〉 ) |  skewY( 〈angle〉 )</code>
 <code>〈color palette〉 = default | 〈name〉 〈color〉 [ , 〈name〉 〈color〉 ]*</code>
+<code>〈filter〉 = brightness( 〈number〉 ) |</code>
+<code>             contrast( 〈number〉 ) |</code>
+<code>             grayscale( 〈number〉 ) |</code>
+<code>             hue-rotate( 〈number〉 ) |</code>
+<code>             invert( 〈number〉 ) |</code>
+<code>             opacity( 〈number〉 ) |</code>
+<code>             saturate( 〈number〉 ) |</code>
+<code>             sepia( 〈number〉 )</code>
 </literallayout>
 
       <para>