serial: sh-sci: Document that serial aliases became optional

Serial aliases are optional since commit 7678f4c2 ("serial:
sh-sci: Add support for dynamic instances").
Update the DT bindings to reflect this.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/renesas,sci-serial.txt

@@ -90,7 +90,7 @@ Required properties:
       - "scif_clk" for the optional external clock source for the frequency
 	divider (SCIF_CLK).
 
-Note: Each enabled SCIx UART should have an alias correctly numbered in the
+Note: Each enabled SCIx UART may have an optional "serialN" alias in the
 "aliases" node.
 
 Optional properties: