Expand system-level synchronization to allow for relationships between arbitrary devices

Following on from a discussion in https://github.com/KhronosGroup/SYCL-Docs/pull/382 which proposed clarification to the wording of memory capability queries. It was discussed (https://github.com/KhronosGroup/SYCL-Docs/pull/382#discussion_r1201798508) that there may be value in expanding the concept of system-level synchronization and the memory scope queries in order to represent platforms where system-level synchronization may be available between some devices and not others.

Start with the discussion and proposed clarification in SYCL-Docs pull request 382, especially discussion 1201798508. Determine the intended scope for representing synchronization and memory-scope relationships between arbitrary devices; the work is done when the required specification changes and behavior are agreed and documented.

Written by the indexing model from the issue text.