Update claims documentation

[rmn30]

When reading the extended temporal safety examples README in #279 it occurred to me that it ought to introduce the idea of claims before talking about them.
This led me to look at https://github.com/CHERIoT-Platform/cheriot-rtos/blob/main/docs/AllocatorClaims.md and I realised it looks like it needs updating, at least to reflect the fact that claims are now merged (in two different instantiations).