[Vala] Adding comments to vapi

["Clément DAVID" <c david86 gmail com>]

I think it may be good to add comments to the VAPI files (generated by
vapigen).

With this improvement we can use doxygen or other tool to get the
documentation with the real Vala case and without devhelp deprecated
file. Furthermore manually written VAPI files maybe more maintainable
and well documented.