[LTP] [PATCH] tst_cmd.h: Convert the rest of comments to kerneldoc

[Petr Vorel]

Hi all,

...
>  Reviewed-by: Li Wang <liwang@redhat.com>

@Li FYI if you put space before RBT it's not taken.

I created a ticket for it:
https://github.com/getpatchwork/patchwork/issues/645

...
> >  #ifdef TST_TEST_H__
> > +/**
> > + * tst_cmd_fds() - :man2:`vfork` + :man2:`execvp` specified program.

execvp is man3, I fixed it before merge.

Thanks for your review, merged!

Kind regards,
Petr