Updated documentation of the clock_mux test case

Signed-off-by: Maciej Kurc <mkurc@antmicro.com>

tests/clock_mux/README.md

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-# `clock mux` tests
-
-This is an example of modeling a clock mux/buffer which has both clock inputs and outputs. According to the "Primitive Block Timing Modeling Tutorial" section "Clock Buffers & Muxes" of the VTR manual (https://docs.verilogtorouting.org/en/latest/tutorials/arch/timing_modeling/#clock-muxes), a clock buffer or a clock mux has to have its outputs(s) as combinational sinks of clock input(s). This contradicts with how regular sequential blocks (like flip-flops) has to be modeled.
-
-In this example the NO_COMB attribute is used. When it's value is forced to 0 then clock inputs may get combinational_sink annotations.

tests/clock_mux/README.rst

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+Clock multiplexing primitive
+++++++++++++++++++++++++++++
+
+This is an example of modeling a clock mux/buffer which has both clock inputs and clock outputs.
+
+According to the "Primitive Block Timing Modeling Tutorial" section "Clock Buffers & Muxes" of the `VTR documentation <https://docs.verilogtorouting.org/en/latest/tutorials/arch/timing_modeling/#clock-muxes>`_, a clock buffer or a clock mux has to have its outputs(s) as combinational sinks of clock input(s). This contradicts with how regular sequential blocks (like flip-flops) have to be modeled.
+
+V2X provides the attribute `(* NO_COMB *)` that when specified on a clock port and given explicitly the value of 0 will prevent it from getting combinational_sink annotations in XML.
+
+.. symbolator:: gmux.sim.v
+
+.. verilog-diagram:: gmux.sim.v
+   :type: netlistsvg
+   :module: GMUX
+
+.. no-license:: gmux.sim.v
+   :language: verilog
+   :caption: tests/clock_mux/gmux.sim.v
+
+In this example the `(* NO_COMB *)` attribute is used on clock inputs IP and IC so they don't get any combinational_sink tags.
+
+.. literalinclude:: gmux.model.xml
+   :language: xml
+   :caption: gmux.model.xml
+
+.. literalinclude:: gmux.pb_type.xml
+   :language: xml
+   :caption: gmux.pb_type.xml
+