counter: Update documentation for new counter registration functions

In order to replace the counter registration API also update the
documentation to the new way.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20211230150300.72196-15-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/generic-counter.rst

@@ -262,11 +262,11 @@ order to communicate with the device: to read and write various Signals
 and Counts, and to set and get the "action mode" and "function mode" for
 various Synapses and Counts respectively.
 
-A defined counter_device structure may be registered to the system by
-passing it to the counter_register function, and unregistered by passing
-it to the counter_unregister function. Similarly, the
-devm_counter_register function may be used if device memory-managed
-registration is desired.
+A counter_device structure is allocated using counter_alloc() and then
+registered to the system by passing it to the counter_add() function, and
+unregistered by passing it to the counter_unregister function. There are
+device managed variants of these functions: devm_counter_alloc() and
+devm_counter_add().
 
 The struct counter_comp structure is used to define counter extensions
 for Signals, Synapses, and Counts.