[DOC] Small doc fix for updates related to _source defaults

[john-wagster]

Community member found an error in the docs related to the update api and the default for _source:
https://elasticstack.slack.com/archives/C018C7B9T5F/p1734000667617069

Quickly validated with this query:

curl -XPOST --header 'Content-Type: application/json' "http://localhost:9200/test/_update/1?_source=true" -d '{
  "doc": {
    "text": "new_name3"
  },
  "doc_as_upsert": true
}'

with _source=true or _source=text in the query params I get:

{
    "_index": "test",
    "_id": "1",
    "_version": 2,
    "result": "noop",
    "_shards": {
        "total": 0,
        "successful": 0,
        "failed": 0
    },
    "_seq_no": 1,
    "_primary_term": 1,
    "get": {
        "_seq_no": 1,
        "_primary_term": 1,
        "found": true,
        "_source": {
            "text": "new_name3"
        }
    }
}

without _source in the query params I get:

{
    "_index": "test",
    "_id": "1",
    "_version": 2,
    "result": "noop",
    "_shards": {
        "total": 0,
        "successful": 0,
        "failed": 0
    },
    "_seq_no": 1,
    "_primary_term": 1
}

This is the correct behavior we want. So I just inverted the language in the docs.

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Status	Branch	Result
✅	8.17
✅	8.x
✅	7.17