tests: vlog: dsp_in_registered: restore comment in model

Signed-off-by: Karol Gugala <kgugala@antmicro.com>

tests/dsp_out_registered/golden.model.xml

diff --git a/tests/dsp_out_registered/golden.model.xml b/tests/dsp_out_registered/golden.model.xml
index 0073b45..e9b223a 100644
--- a/tests/dsp_out_registered/golden.model.xml
+++ b/tests/dsp_out_registered/golden.model.xml
@@ -1,4 +1,5 @@
 <models xmlns:xi="http://www.w3.org/2001/XInclude">
+  <!-- https://docs.verilogtorouting.org/en/latest/tutorials/arch/timing_modeling/#mixed-sequential-combinational-block -->
   <xi:include href="../dsp_combinational/dsp_combinational.model.xml" xpointer="xpointer(models/child::node())"/>
   <xi:include href="../fig42-dff/dff.model.xml" xpointer="xpointer(models/child::node())"/>
 </models>