[CPP-418] Mention that GNU vector initializers are syntactically

similar to array initializers.

cpp/ql/src/semmle/code/cpp/exprs/Literal.qll

@@ -310,6 +310,9 @@ class ArrayAggregateLiteral extends ArrayOrVectorAggregateLiteral {
 
 /**
  * A C/C++ aggregate literal that initializes a GNU vector type.
+ *
+ * Braced initializer lists are used, similarly to what is done
+ * for arrays.
  * ```
  * typedef int v4si __attribute__ (( vector_size(4*sizeof(int)) ));
  * v4si v = (v4si){ 1, 2, 3, 4 };