iio: ABI: stm32-timer-trigger: Fuse unusual ABI into main doc.

We can't duplicate the description of sampling_frequency. This device
has some unusual requirements which we solved by giving a
sampling_frequency of 0 special meaning. As such add a note
about this unusual usage to the main documentation.

Whilst I don't particularly like this resolution, it is the best
I could come up with given earlier discussion on this topic.

Link: https://lore.kernel.org/linux-iio/[e-mail]/
Signed-off-by: Jonathan Cameron <[e-mail]>
Cc: Fabrice Gasnier <[e-mail]>
Reviewed-by: Mauro Carvalho Chehab <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/ABI/testing/sysfs-bus-iio

···
 		relevant directories.  If it affects all of the above
 		then it is to be found in the base device directory.
 
+		The stm32-timer-trigger has the additional characteristic that
+		a sampling_frequency of 0 is defined to stop sampling.
+
 What:		/sys/bus/iio/devices/iio:deviceX/sampling_frequency_available
 What:		/sys/bus/iio/devices/iio:deviceX/in_intensity_sampling_frequency_available
 What:		/sys/bus/iio/devices/iio:deviceX/in_proximity_sampling_frequency_available

Documentation/ABI/testing/sysfs-bus-iio-timer-stm32

···
 		Reading returns the current master modes.
 		Writing set the master mode
 
-What:		/sys/bus/iio/devices/triggerX/sampling_frequency
-KernelVersion:	4.11
-Contact:	[e-mail]
-Description:
-		Reading returns the current sampling frequency.
-		Writing an value different of 0 set and start sampling.
-		Writing 0 stop sampling.
-
 What:		/sys/bus/iio/devices/iio:deviceX/in_count0_preset
 KernelVersion:	4.12
 Contact:	[e-mail]