iio: Improve the kernel-doc of iio_trigger_poll

Move the kernel-doc of the function to industrialio-trigger.c
Add a note on the context where the function is expected to be called.

Signed-off-by: Mehdi Djait <mehdi.djait.k@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/bd84fc17e9d22eab998bf48720297f9a77689f45.1677761379.git.mehdi.djait.k@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>