lib: json: clarify JSON_OBJ_DESCR_ARRAY usage

This (and JSON_OBJ_DESCR_ARRAY_NAMED) are really intended for handling
arrays of primitive type only. They don't allow users to declare
descriptors for arrays of objects. Clarify this in the Doxygen.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>