staging:iio:documentation: document filter_low_pass_3db_frequency.

Add the sysfs ABI docs for all current instances of this
parameter.
Signed-off-by: NJonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/staging/iio/Documentation/sysfs-bus-iio

@@ -276,6 +276,16 @@ Description:
 		If a discrete set of scale values are available, they
 		are listed in this attribute.
 
+What:		/sys/.../in_accel_filter_low_pass_3db_frequency
+What:		/sys/.../in_magn_filter_low_pass_3db_frequency
+What:		/sys/.../in_anglvel_filter_low_pass_3db_frequency
+KernelVersion:	3.2
+Contact:	linux-iio@vger.kernel.org
+Description:
+		If a known or controllable low pass filter is applied
+		to the underlying data channel, then this parameter
+		gives the 3dB frequency of the filter in Hz.
+
 What:		/sys/bus/iio/devices/iio:deviceX/out_voltageY_raw
 KernelVersion:	2.6.37
 Contact:	linux-iio@vger.kernel.org