[Architecture File] Add comments for titan architecture to clarify the choice of pin equivalence

Author: tangxifan

vtr_flow/arch/titan/stratixiv_arch.timing.xml

@@ -4471,6 +4471,15 @@
         <input name="control_in" num_pins="7" equivalent="full"/>
         <input name="cin" num_pins="1" equivalent="none"/>
         <input name="sharein" num_pins="1" equivalent="none"/>
+        <!-- Stratix IV does have instance equivalence only for the data_out port
+             where ALM outputs can be swapped to reach the most suitable pin location 
+             for routers. However, it may require significant changes to packing results
+             which is not supported yet.
+             Therefore, we assume full equivalence here, so that ALM outputs can be swapped
+             through a local routing architecture without changing packing results.
+             Please note that this may cause optimistic routing results, being
+             better in routability than what actual hardware can support
+          -->
         <output name="data_out" num_pins="40" equivalent="full"/>
         <output name="cout" num_pins="1" equivalent="none"/>
         <output name="shareout" num_pins="1" equivalent="none"/>