[TF-M] [RFC] Updated mailbox agent API documents

Hi,

The mailbox agent API was proposed before and this time more implementation details are introduced in the document: Docs: Implementation details for Mailbox Agent API (Ifec409eb) * Gerrit Code Review (trustedfirmware.org)https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/20421

Feel free to comment, and feel free to create implementations for it - remember response to this thread before you do.

Thanks.

/Ken