Correct the -gtk-icon-filter docs

I was overlooking that it is possible to specify multiple
functions. Also add a reference to the relevant spec draft.

docs/reference/gtk/css-properties.xml

@@ -445,12 +445,12 @@ We use <literallayout> for syntax productions, and each line is put in a <code>
             </row>
             <row>
               <entry>‑gtk‑icon‑filter</entry>
-              <entry><code>none | 〈filter〉</code></entry>
+              <entry><code>none | 〈filter-function-list〉</code></entry>
               <entry><code>none</code></entry>
               <entry>✓</entry>
               <entry></entry>
               <entry></entry>
-              <entry></entry>
+              <entry><ulink url="https://drafts.fxtf.org/filters/#FilterProperty">CSS3</ulink></entry>
             </row>
           </tbody>
         </tgroup>
@@ -466,6 +466,7 @@ 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-function-list〉 = [ 〈filter〉]+</code>
 <code>〈filter〉 = brightness( 〈number〉 ) |</code>
 <code>             contrast( 〈number〉 ) |</code>
 <code>             grayscale( 〈number〉 ) |</code>