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]

LGTM

[github-actions]

The backport to 9.0 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-9.0 9.0
# Navigate to the new working tree
cd .worktrees/backport-9.0
# Create a new branch
git switch --create backport-4705-to-9.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 30931cbfa17b7c3bfafdeafbe6deb00a7bdb704e
# Push it to GitHub
git push --set-upstream origin backport-4705-to-9.0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-9.0

Then, create a pull request where the base branch is 9.0 and the compare/head branch is backport-4705-to-9.0.