Documentation: ABI: add events sampling frequency in sysfs-bus-iio

Documentation: ABI: add events sampling frequency in sysfs-bus-iio

Some devices have an internal clock used by the events to space the
conversions.
The max1363 introduced the option in
commit 168c9d95a940 ("iio:adc:max1363 move from staging.")
and ad799x in
commit ba1d79613df3 ("staging:iio:ad799x: Use event spec for threshold
hysteresis")

Signed-off-by: Jorge Marques <jorge.marques@analog.com>
Link: https://patch.msgid.link/20250321-abi-oversampling-events-frequency-v1-1-794c1ab2f079@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>