[flang][doc] Added remark about array element references in data clauses. (#72332)

Author: vzakhari

flang/docs/OpenACC.md

@@ -25,3 +25,10 @@ local:
   logical expression.
 * `!$acc routine` directive can be placed at the top level. 
 * `!$acc cache` directive accepts scalar variable.
+
+## Remarks about incompatibilities with other implementations
+* Array element references in the data clauses are equivalent to array sections
+  consisting of this single element, i.e. `copyin(a(n))` is equivalent to
+  `copyin(a(n:n))`.  Some other implementations have treated it as
+  `copyin(a(:n))`, which does not correspond to OpenACC spec – Flang does not
+  support this interpretation of an array element reference.