extcon: Amend kernel documentation of struct extcon_dev

First of all, the @lock description is missing. Add it.
Second, correct the terminator value for the mutual exclusive
cabling.

Signed-off-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Chanwoo Choi <[e-mail]>

drivers/extcon/extcon.h

···
  *			are disabled.
  * @mutually_exclusive:	Array of mutually exclusive set of cables that cannot
  *			be attached simultaneously. The array should be
- *			ending with NULL or be NULL (no mutually exclusive
- *			cables). For example, if it is { 0x7, 0x30, 0}, then,
+ *			ending with 0 or be NULL (no mutually exclusive cables).
+ *			For example, if it is {0x7, 0x30, 0}, then,
  *			{0, 1}, {0, 1, 2}, {0, 2}, {1, 2}, or {4, 5} cannot
  *			be attached simulataneously. {0x7, 0} is equivalent to
  *			{0x3, 0x6, 0x5, 0}. If it is {0xFFFFFFFF, 0}, there
···
  * @nh:			Notifier for the state change events from this extcon
  * @entry:		To support list of extcon devices so that users can
  *			search for extcon devices based on the extcon name.
- * @lock:
+ * @lock:		Protects device state and serialises device registration
  * @max_supported:	Internal value to store the number of cables.
  * @extcon_dev_type:	Device_type struct to provide attribute_groups
  *			customized for each extcon device.