dt-bindings: i3c: Document dropped support for I2C 10 bit devices

Because this patch series dropped support for 10 bit I2C devices, I'm
documenting this.
Signed-off-by: Przemyslaw Gaj <pgaj@cadence.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

Documentation/devicetree/bindings/i3c/i3c.txt

@@ -39,7 +39,9 @@ valid here, but several new properties have been added.
 New constraint on existing properties:
 --------------------------------------
 - reg: contains 3 cells
-  + first cell : still encoding the I2C address
+  + first cell : still encoding the I2C address. 10 bit addressing is not
+    supported. Devices with 10 bit address can't be properly passed through
+    DEFSLVS command.
 
   + second cell: shall be 0