Convert ntoskrnl documentation into Doxygen format

There are a lot of functions in the kernel, which have documentation headers, but it's not in Doxygen format so you can't see it on https://doxygen.reactos.org in convenient format.

Most of the documented functions are in Ex, Io and Ob subsystems.

https://reactos.org/wiki/Coding_Style and http://www.doxygen.nl/manual/commands.html can be used as a reference