update documentation on the through channel
@@ -37,6 +37,8 @@ Each ``<pb_type>`` should contain a ``<mode>`` that describe the physical implem
.. warning:: Do NOT enable if you are not using the tileable routing resource graph generator!
+ .. warning:: Current through channel supports only a fixed routing channel!
+
``<switch_block>`` may include addition syntax to enable different connectivity for pass tracks
.. option:: sub_type="<string>"
