[DOCS] copy_to only works one level deep, not recursively

[geekpete]

Clarify that you cannot "chain" fields with copy_to and expect it to recursively copy via intermediary fields.

[elasticmachine]

Pinging @elastic/es-search

[geekpete]

I also had an example snippet to demonstrate this scenario but thought that single bullet point describing it was enough:

# cleanup from previous test runs
DELETE my_index

# create index with copy_to for field_1->field_2, another for field_2->field_3
PUT my_index
{
  "settings": {
    "number_of_shards": 1,
    "number_of_replicas": 0
  }, 
  "mappings": {
    "_doc": {
      "properties": {
        "field_1": {
          "type": "text",
          "copy_to": "field_2"
        },
        "field_2": {
          "type": "text",
          "copy_to": "field_3" 
        },
        "field_3": {
          "type": "text",
          "store": true
        }
      }
    }
  }
}

# index a doc with a value in field_1
PUT my_index/_doc/1
{
  "field_1": "testing field 1"
}

# index another doc with a value in field_2
PUT my_index/_doc/2
{
  "field_2": "testing field 2"
}


# doc 1 doesn't copy the value into field_3, doc 2 does.
GET my_index/_search
{
  "stored_fields": [ "field_1","field_2","field_3" ],
  "_source": "*"
}

Resulting in:

{
  "took" : 0,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : 2,
    "max_score" : 1.0,
    "hits" : [
      {
        "_index" : "my_index",
        "_type" : "_doc",
        "_id" : "1",
        "_score" : 1.0,
        "_source" : {
          "field_1" : "testing field 1"
        }
      },
      {
        "_index" : "my_index",
        "_type" : "_doc",
        "_id" : "2",
        "_score" : 1.0,
        "_source" : {
          "field_2" : "testing field 2"
        },
        "fields" : {
          "field_3" : [
            "testing field 2"
          ]
        }
      }
    ]
  }
}

[cbuescher]

@geekpete thanks, LGTM. Which version do you thing this addition should be targeted at? Do you want to merge or should I do that

[geekpete]

6.x and 7.x versions of the docs I guess would be great.
I can merge and cherry-picky to those versions.

[cbuescher]

@geekpete thanks for merging, I also cherry-picked to 6.x and 6.6 since I think they were missed, or did you leave them out on purpose?

[geekpete]

Had just missed them not left out on purpose.
Will be looking to use the backport tool by sqren next time I cherry pick across branches.