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
  access" to USM.  Add rules to explain this.

* Replace the term "unified addressing", which was poorly defined, with
  a new term "stable addresses", and clarify exactly what guarantees we
  provide about uniqueness of pointers for the three USM allocation
  types.

Closes KhronosGroup#186.

Author: gmlueck

adoc/chapters/architecture.adoc

@@ -1067,6 +1067,7 @@ values:
     work-item;
   * [code]#sycl::memory_scope::device# The ordering constraint applies only
     to work-items executing on the same device as the calling work-item;
+    TODO: Should this include sub-devices also?
   * [code]#sycl::memory_scope::system# The ordering constraint applies to any
     work-item or host thread in the system that is currently permitted to
     access the memory allocation containing the referenced object, as