Clarify Data class methods semantics

[traversaro]

Content to be added, opening it just as a remainder.

[traversaro]

This is already documented:

         * @note the elements of the lowerBound and upperBound are not copied inside the library.
         * The user has to guarantee that the lifetime of the objects passed is the same of the
         * OsqpEigen object

I was affected by a bug in which I was passing a temporary Eigen vector to setBounds in robotology/wb-toolbox#209, but apparently this is correctly document, even if a bit surprising, but I guess the copies are avoided for performances reason.