Docs: Implementation details for Mailbox Agent API

The purpose is to cover as much detail as could, and here are
the contained items:

- API implementation.
- Partition-level operations for agents.
- IDLE processing.

Signed-off-by: Ken Liu <Ken.Liu@arm.com>
Change-Id: Ifec409eb059d0a3928bb8ff1d20683f656ef2673
1 file changed