add note

Author: yaacovCR

spec/Section 6 -- Execution.md

@@ -375,7 +375,13 @@ Unsubscribe(responseStream):
 If an operation contains `@defer` or `@stream` directives, execution may also
 result in an Subsequent Result stream in addition to the initial response. The
 Subsequent Results may be published lazily as requested by a Publisher, whose
-state is held in a Publisher Record.
+state is held in a Publisher Record and within Subsequent Result and Incremental
+Data Records referenced therein. Details about the records are listed below,
+following which the algorithms accessing and manipulating this state are given.
+
+Note: To mitigate concurrency-related issues, it is recommended that the
+Publisher ensures all operations that access and manipulate incremental delivery
+state are performed atomically.
 
 ### Publisher Records: