[TF-M] [RFC] Update on Secure Partition Runtime Library design document

Hi,

I was trying to fix some typos in this document before, so I just create a short commit; but after couple times of updating I added more content into it, so called a review on this part.

The changed part is, involving the RoT Service API implementation into SPRTL to simplify the caller usage. The RoT Service API is proposed in PSA FF 1.0.0, and I think this is the ideal place for putting it.

The link: https://review.trustedfirmware.org/c/trusted-firmware-m/+/1886

Thanks.

/Ken