Clarify which atomic operations synchronize

The ISO C++ synchronizes-with relationship does not account for scopes.
The scopes do not need to match exactly, but there are restrictions on which
pairs of scopes are valid.

This is the final part of the clarification for work-item scope atomics;
a work-item scope atomic cannot sychronize with the atomic operations
performed by other work-items, and so their effects are not guaranteed to be
visible to other work-items without some other synchronization taking place.

Author: Pennycook

adoc/chapters/architecture.adoc

@@ -1099,6 +1099,12 @@ programming and to provide a meaningful way to describe the behavior of
 Potentially concurrent conflicting actions with different memory scopes are
 considered a data race, resulting in undefined behavior.
 
+An atomic operation _A_ with scope _S~1~_ can only synchronize with another
+atomic operation _B_ with scope _S~2~_ if:
+
+* The work-item which executed _A_ is in both _S~1~_ and _S~2~_; and
+* The work-item which executed _B_ is in both _S~1~_ and _S~2~_.
+
 The memory scopes are listed above from narrowest
 ([code]#memory_scope::work_item#) to widest ([code]#memory_scope::system#).
 
@@ -1119,10 +1125,9 @@ supplied.
 ====
 The addition of memory scopes to the {cpp} memory model modifies the definition
 of some concepts from the {cpp} core language.
-For example: the synchronizes-with relationship and sequential consistency must
-be defined in a way that accounts for atomic operations with differing (but
-compatible) scopes, in a manner similar to the <<opencl20, OpenCL 2.0
-specification>>.
+For example: sequential consistency must be defined in a way that accounts for
+atomic operations with differing (but compatible) scopes, in a manner similar to
+the <<opencl20, OpenCL 2.0 specification>>.
 Efforts to formalize the memory model of SYCL are ongoing, and a formal memory
 model will be included in a future version of the SYCL specification.
 ====