iio:ABI docs: Move specific description of out_altvoltageX_frequency to main docs.

The adf4371 has channels that are very closely coupled, so additional
documentation is needed to express these constraints.
Unfortunately having the same sysfs filename in multiple documentation
does not work well when generating automated documentation.
To avoid this issue, we add a new device specific description to the
main docs and remove the one in the device specific file.

Fixes
$ scripts/get_abi.pl validate
Warning: /sys/bus/iio/devices/iio:deviceX/out_altvoltageY_frequency is defined 2 times:  ./Documentation/ABI/testing/sysfs-bus-iio-frequency-adf4371:0  ./Documentation/ABI/testing/sysfs-bus-iio:599

Cc: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reported-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20210117153816.696693-8-jic23@kernel.org

Documentation/ABI/testing/sysfs-bus-iio

@@ -665,6 +665,25 @@ Description:
 		Output frequency for channel Y in Hz. The number must always be
 		specified and unique if the output corresponds to a single
 		channel.
+		Some drivers have additional constraints:
+		ADF4371 has an integrated VCO with fundamendal output
+		frequency ranging from 4000000000 Hz 8000000000 Hz.
+
+		out_altvoltage0_frequency:
+			A divide by 1, 2, 4, 8, 16, 32 or circuit generates
+			frequencies from 62500000 Hz to 8000000000 Hz.
+		out_altvoltage1_frequency:
+			This channel duplicates the channel 0 frequency
+		out_altvoltage2_frequency:
+			A frequency doubler generates frequencies from
+			8000000000 Hz to 16000000000 Hz.
+		out_altvoltage3_frequency:
+			A frequency quadrupler generates frequencies from
+			16000000000 Hz to 32000000000 Hz.
+
+		Note: writes to one of the channels will affect the frequency of
+		all the other channels, since it involves changing the VCO
+		fundamental output frequency.
 
 What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_phase
 KernelVersion:	3.4.0

Documentation/ABI/testing/sysfs-bus-iio-frequency-adf4371

-What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_frequency
-KernelVersion:
-Contact:	linux-iio@vger.kernel.org
-Description:
-		Stores the PLL frequency in Hz for channel Y.
-		Reading returns the actual frequency in Hz.
-		The ADF4371 has an integrated VCO with fundamendal output
-		frequency ranging from 4000000000 Hz 8000000000 Hz.
-
-		out_altvoltage0_frequency:
-			A divide by 1, 2, 4, 8, 16, 32 or circuit generates
-			frequencies from 62500000 Hz to 8000000000 Hz.
-		out_altvoltage1_frequency:
-			This channel duplicates the channel 0 frequency
-		out_altvoltage2_frequency:
-			A frequency doubler generates frequencies from
-			8000000000 Hz to 16000000000 Hz.
-		out_altvoltage3_frequency:
-			A frequency quadrupler generates frequencies from
-			16000000000 Hz to 32000000000 Hz.
-
-		Note: writes to one of the channels will affect the frequency of
-		all the other channels, since it involves changing the VCO
-		fundamental output frequency.
-
 What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_name
 KernelVersion:
 Contact:	linux-iio@vger.kernel.org