[intro.races] Clarify 'synchronization operation'.

Fixes #1611 and CWG 2297.

Author: jensmaurer

source/intro.tex

@@ -1198,9 +1198,12 @@
 memory location.
 
 \pnum
-The library defines a number of atomic operations (Clause~\ref{atomics}) and
-operations on mutexes (Clause~\ref{thread}) that are specially identified as
-synchronization operations. These operations play a special role in making
+The library identifies certain operations as
+\defnx{synchronization operations}{synchronization operation}, as specified
+in the \sync element
+(\ref{support.signal}, \ref{util.smartptr.shared.atomic}, \ref{atomics.order},
+\ref{atomics.fences}, Clause~\ref{thread}).
+These operations play a special role in making
 assignments in one thread visible to another. A synchronization operation on one
 or more memory locations is either a consume operation, an acquire operation, a
 release operation, or both an acquire and release operation. A synchronization