add note about triggered buffers in dox source
add note about triggered buffers in dox source

add doc on triggered buffers needed triggers set up.

Signed-off-by: Robin Getz <c66e5a86ef9317cf35cecb2ed063996b2100ed4b@analog.com>
analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio