[OpenAPI] Edit mget, multi term vectors, and terms enum APIs

[lcawl]

Relates to #2635

This PR adds a summary and description for the mget, multi term vector, and terms enum APIs (copied from https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-multi-get.html, https://www.elastic.co/guide/en/elasticsearch/reference/master/docs-multi-termvectors.html, and https://www.elastic.co/guide/en/elasticsearch/reference/master/search-terms-enum.html).
It also changes their tags from "mget", "mtermvectors", and "terms_enum" to "search" so that they're grouped with the other document APIs.

[github-actions]

Following you can find the validation results for the API you have changed.

API	Status	Request	Response
mget	🟢	51/51	50/50

You can validate this API yourself by using the make validate target.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
mget	🟢	51/51	50/50
mtermvectors	🟢	10/10	10/10

You can validate these APIs yourself by using the make validate target.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
mget	🟢	51/51	50/50
mtermvectors	🟢	10/10	10/10

You can validate these APIs yourself by using the make validate target.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
delete_by_query_rethrottle	🟢	1/1	1/1
mget	🟢	51/51	50/50
mtermvectors	🟢	10/10	10/10
reindex_rethrottle	🟢	2/2	2/2
update_by_query_rethrottle	🟢	1/1	1/1

You can validate these APIs yourself by using the make validate target.

[karenzone]

LGTM

[github-actions]

The backport to 8.x 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-8.x 8.x
# Navigate to the new working tree
cd .worktrees/backport-8.x
# Create a new branch
git switch --create backport-3052-to-8.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 bfbe862fceb2dc6c4d3379646885f1efd650fdcd
# Push it to GitHub
git push --set-upstream origin backport-3052-to-8.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.x

Then, create a pull request where the base branch is 8.x and the compare/head branch is backport-3052-to-8.x.

[github-actions]

The backport to 8.16 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-8.16 8.16
# Navigate to the new working tree
cd .worktrees/backport-8.16
# Create a new branch
git switch --create backport-3052-to-8.16
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 bfbe862fceb2dc6c4d3379646885f1efd650fdcd
# Push it to GitHub
git push --set-upstream origin backport-3052-to-8.16
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.16

Then, create a pull request where the base branch is 8.16 and the compare/head branch is backport-3052-to-8.16.