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/20210315101217.00002c50@Huawei.com/


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/20220626165511.602202-4-jic23@kernel.org