iio: buffer: document that buffer callback must be context safe

iio: buffer: document that buffer callback must be context safe

Document that the callback registered with iio_channel_get_all_cb()
must be safe to call from any context since it is called from by
iio_push_to_buffer() which can be called in any context.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>