habanalabs: add comments in uapi/misc/habanalabs.h

Add comment about minimum and maximum size of command buffer.
Add some text about the expected input of CS IOCTL.

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>