Updated qldoc for ArrayUpdate

Author: artem-smotrakov

java/ql/src/experimental/semmle/code/java/security/StaticInitializationVectorQuery.qll

@@ -28,7 +28,12 @@ private class StaticByteArrayCreation extends ArrayCreationExpr {
   }
 }
 
-/** Defines a sub-set of expressions that update an array. */
+/**
+ * Defines a sub-set of expressions that update either content of an array or an array reference.
+ * There sub-set covers only methods that are likely to set a non-static IV.
+ * For example, `java.util.Arrays.fill()` is not covered because it assigns the same value
+ * to each element of the array.
+ */
 private class ArrayUpdate extends Expr {
   Expr array;