Fix #78370: imagefilltoborder does not fill

Author: cmb69

reference/image/functions/imagefilltoborder.xml

@@ -108,6 +108,18 @@ imagedestroy($im);
    </example>
   </para>
  </refsect1>
+
+ <refsect1 role="notes">
+  &reftitle.notes;
+  <simpara>
+   The algorithm does not explicitly remember which pixels have
+   already been set, but rather infers that from the color of the
+   pixel, so it cannot distinguish between freshly set pixels and
+   pixels that are already there.  That means chosing any fill color
+   that is already used in the image may yield undesired results.
+  </simpara>
+ </refsect1>
+
 </refentry>
 
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