Add link to _update_by_query API examples

[charlotte-hoblik]

• This PR adds a link for the _update_by_query API examples to the Elasticsearch reference documentation, as part of the approach to unify API reference documentation.
• Adds 3 paragraphs to the API description as this information seemingly was missing from the API docs Requires confirmation!

Link to 8.18 documentation: https://www.elastic.co/guide/en/elasticsearch/reference/8.18/docs-update-by-query.html

DO NOT MERGE BEFORE elastic/elasticsearch#130273

Connected to:

https://github.com/elastic/docs-projects/issues/302
elastic/elasticsearch#130273

[leemthompo]

Adds 3 paragraphs to the API description as this information seemingly was missing from the API docs Requires confirmation!

• a link to old docs would be helpful to confirm
• it would also be nice to see a screenshot of the previewed description update to ensure formatting is as expected :)

[charlotte-hoblik]

Thank you @leemthompo
I've added the link to the old docs to the description as well: https://www.elastic.co/guide/en/elasticsearch/reference/8.18/docs-update-by-query.html

Screenshot of the updated description:

[leemthompo]

How come we're losing this section?