Add definition of when a USM allocation is 'accessible'.

[VerenaBeckham]

Part 2 of addressing #186.

[TApplencourt]

Should we replace this with something like

A USM allocation in only accessible in devices associated with his <<context>>. More restriction apply:

And then remove the context part of both shared and host? If not we need to add a "context" for device alloc:

A USM device allocation is accessible only from the device and the context specified in the USM allocation.

Because, AFAIK, the code bellow is UB

sycl::context C1{D} C2{D}
sycl::queue Q1(C1,D); Q2(C2,D);
auto p = mallloc_device(Q1);
Q2.submit( *p); // UB. Not the same context, even if it's the same "physical" device

[VerenaBeckham]

Ah, you are correct. I prefer folding the Context requirement into the bullet points, because it does not apply to the host allocations.
I made that change. I hope it is clear enough to say "device and context".
I also added that device and shared allocations have an associated device, which I think is good info. I've had to look up the different malloc APIs to understand the difference.