net: netconsole: Add description for 'netconsole_target's extended attribute

Fixes the following W=1 kernel build warning(s):

 drivers/net/netconsole.c:104: warning: Function parameter or member 'extended' not described in 'netconsole_target'
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201102114512.1062724-29-lee.jones@linaro.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/netconsole.c

@@ -83,6 +83,7 @@ static struct console netconsole_ext;
  *		whether the corresponding netpoll is active or inactive.
  *		Also, other parameters of a target may be modified at
  *		runtime only when it is disabled (enabled == 0).
+ * @extended:	Denotes whether console is extended or not.
  * @np:		The netpoll structure for this target.
  *		Contains the other userspace visible parameters:
  *		dev_name	(read-write)