[8.x][OpenAPI] Edit mget, multi term vectors, and terms enum APIs (#3052)

[lcawl]

Backports #3052 with bfbe862

[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.

[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.