Clarify USM behavior

Clarify several things about USM:

* We did not have precise rules about when USM memory allocated for
  one device could be accessed by another device.  This is now
  clarified, and a few new device aspects were added to tell
  applications when this is allowed.

* We did not have precise rules about sharing a USM memory region
  between two devices when those devices do not support "concurrent
  atomic" accesses to USM.  Add rules to explain this.

* We were not clear about the role of the context that is used when
  allocating USM memory.  Clarify how this affects the accessibility
  of USM memory on various devices.

* Clarify that the device returned by `get_pointer_device` is the same
  device used when allocating the USM memory region.

Closes internal issue 599.
Partially addresses KhronosGroup#184.
Closes KhronosGroup#186.

Author: gmlueck

adoc/chapters/glossary.adoc

@@ -145,6 +145,10 @@ object. For the full description please refer to <<subsec:buffers>>.
     the text of any backend specifications and excluding the text for any
     extensions.
 
+[[descendent-device]]descendent device::
+    The descendent devices of device _D_ include all of the sub-devices of _D_,
+    all of the sub-devices of those devices, etc.
+
 [[device]]device::
     A SYCL device is an abstraction of a piece of hardware that can execute
     <<sycl-kernel-function,SYCL kernels>>.