[docs] Add missed examples for datasources.

examples/data-sources/elasticsearch_host/data-source.tf

@@ -0,0 +1,3 @@
+data "elasticsearch_host" "test" {
+  active = true
+}

examples/data-sources/elasticsearch_opendistro_destination/data-source.tf

@@ -0,0 +1,16 @@
+# Example destination in other terraform plan
+# resource "elasticsearch_opendistro_destination" "test" {
+#   body = <<EOF
+# {
+#   "name": "my-destination",
+#   "type": "slack",
+#   "slack": {
+#     "url": "http://www.example.com"
+#   }
+# }
+# EOF
+# }
+
+data "elasticsearch_opendistro_destination" "test" {
+  name = my-destination"
+}