kobject: Improve docs for kobject_add/del

There is currently some confusion on how to wind back
kobject_init_and_add() during the error paths in code that uses this
function.

Add documentation to kobject_add() and kobject_del() to help clarify the
usage.

Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>