C++: update change note per suggestion from peer review

Author: andersfugmann

cpp/change-notes/2021-09-13-overflow-static.md

@@ -1,6 +1,4 @@
 lgtm,codescanning
-* The `Buffer` library considers more fields to be of variable size
-  for array members of size 0 or 1. Buffer size calculation of array type
-  fields of size 0 or 1 in unions are considered pointers to the union
-  and will return the size of the union itself. The changes reduces
-  the number of false positives in cpp/static-buffer-overflow
+* The `memberMayBeVarSize` predicate considers more fields to be variable size.
+  As a result, the "Static buffer overflow" query (cpp/static-buffer-overflow)
+  produces fewer false positives.