doc: iio: ad4695: describe oversampling support

Add a section to the ad4695 documentation describing how to use the
oversampling feature. Also add some clarification on how the
oversampling ratio influences effective sample rate in the offload
section.

Signed-off-by: Trevor Gamblin <[e-mail]>
Tested-by: David Lechner <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/iio/ad4695.rst

···
 System calibration is supported using the channel gain and offset registers via
 the ``calibscale`` and ``calibbias`` attributes respectively.
 
+Oversampling
+------------
+
+The chip supports per-channel oversampling when SPI offload is being used, with
+available oversampling ratios (OSR) of 1 (default), 4, 16, and 64.  Enabling
+oversampling on a channel raises the effective number of bits of sampled data to
+17 (OSR == 4), 18 (16), or 19 (64), respectively. This can be set via the
+``oversampling_ratio`` attribute.
+
+Setting the oversampling ratio for a channel also changes the sample rate for
+that channel, since it requires multiple conversions per 1 sample. Specifically,
+the new sampling frequency is the PWM sampling frequency divided by the
+particular OSR. This is set automatically by the driver when setting the
+``oversampling_ratio`` attribute. For example, if the device's current
+``sampling_frequency`` is 10000 and an OSR of 4 is set on channel ``voltage0``,
+the new reported sampling rate for that channel will be 2500 (ignoring PWM API
+rounding), while all others will remain at 10000.  Subsequently setting the
+sampling frequency to a higher value on that channel will adjust the CNV trigger
+period for all channels, e.g. if ``voltage0``'s sampling frequency is adjusted
+from 2500 (with an OSR of 4) to 10000, the value reported by
+``in_voltage0_sampling_frequency`` will be 10000, but all other channels will
+now report 40000.
+
+For simplicity, the sampling frequency of the device should be set (considering
+the highest desired OSR value to be used) first, before configuring oversampling
+for specific channels.
+
 Unimplemented features
 ----------------------
 
 - Additional wiring modes
 - Threshold events
-- Oversampling
 - GPIO support
 - CRC support
 
···
 divided by the number of enabled channels. So if 4 channels are enabled, with
 the ``in_voltageY_sampling_frequency`` attributes set to 1 MHz, the effective
 sample rate is 250 kHz.
+
+With oversampling enabled, the effective sample rate also depends on the OSR
+assigned to each channel. For example, if one of the 4 channels mentioned in the
+previous case is configured with an OSR of 4, the effective sample rate for that
+channel becomes (1 MHz / 4 ) = 250 kHz. The effective sample rate for all
+four channels is then 1 / ( (3 / 1 MHz) + ( 1 / 250 kHz) ) ~= 142.9 kHz. Note
+that in this case "sample" refers to one read of all enabled channels (i.e. one
+full cycle through the auto-sequencer).