docs/rds_cluster_instance: the engine/version should match that of th…

…e cluster (#14037)

If you try to create an RDS instance with a different engine/version to
the cluster, you get an InvalidParameterCombination error from Terraform
and no instances.

Changing the behaviour so that instances inherit their config from the
cluster is a big change.

This patch just tweaks the documented examples, so if people copy/paste
the example and change the engine/version of the cluster, it will still
work correctly.  It also highlights that the engine/version should
match between the instance and the cluster.

Relates to #10585

website/docs/r/rds_cluster_endpoint.html.markdown

@@ -30,20 +30,26 @@ resource "aws_rds_cluster_instance" "test1" {
   cluster_identifier = "${aws_rds_cluster.default.id}"
   identifier         = "test1"
   instance_class     = "db.t2.small"
+  engine             = "${aws_rds_cluster.default.engine}"
+  engine_version     = "${aws_rds_cluster.default.engine_version}"
 }
 
 resource "aws_rds_cluster_instance" "test2" {
   apply_immediately  = true
   cluster_identifier = "${aws_rds_cluster.default.id}"
   identifier         = "test2"
   instance_class     = "db.t2.small"
+  engine             = "${aws_rds_cluster.default.engine}"
+  engine_version     = "${aws_rds_cluster.default.engine_version}"
 }
 
 resource "aws_rds_cluster_instance" "test3" {
   apply_immediately  = true
   cluster_identifier = "${aws_rds_cluster.default.id}"
   identifier         = "test3"
   instance_class     = "db.t2.small"
+  engine             = "${aws_rds_cluster.default.engine}"
+  engine_version     = "${aws_rds_cluster.default.engine_version}"
 }
 
 resource "aws_rds_cluster_endpoint" "eligible" {

website/docs/r/rds_cluster_instance.html.markdown

@@ -31,6 +31,8 @@ resource "aws_rds_cluster_instance" "cluster_instances" {
   identifier         = "aurora-cluster-demo-${count.index}"
   cluster_identifier = "${aws_rds_cluster.default.id}"
   instance_class     = "db.r4.large"
+  engine             = "${aws_rds_cluster.default.engine}"
+  engine_version     = "${aws_rds_cluster.default.engine_version}"
 }
 
 resource "aws_rds_cluster" "default" {