Clarify USM allocation accessibility by other devices

In §4.8.2 where the different kinds of USM are described table 99 the characteristics of the different kinds of USM allocation. Part of the table describes whether a USM allocation is accessible by another device. This is described for device and shared allocations as `Optional`, which I interpret to mean that it's implementation defined whether that is supported. However, for device allocations it also has `P2P` in brackets, while for shared allocations it doesn't, though the term `P2P` or `peer-to-peer` doesn't seem to be referenced anywhere else in the specification.

I was hoping to clarify in which circumstances a device or shared memory allocation can be expected to be accessible by other devices, i.e. is there a way to query this, and what is meant by `P2P` and what how this differs between device and shared allocations.

cc @jbrodman

Start with §4.8.2 and Table 99 in the SYCL specification, then review the surrounding USM allocation terminology and the issue discussion. Clarify what “Optional” and “P2P” mean for device and shared allocations, including whether accessibility can be queried, and update the specification so the expected circumstances are unambiguous.

Written by the indexing model from the issue text.