Doxyfile.in: capture warnings when creating libiio api doc
Doxyfile.in: capture warnings when creating libiio api doc

And eventually check it on CI to make sure we are documenting
everything.

Signed-off-by: Robin Getz <c66e5a86ef9317cf35cecb2ed063996b2100ed4b@analog.com>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio