[DOCS] Adds description to global APIs part 2

[szabosteve]

Overview

Related to #964.

Adds descriptions to the:

• create
• delete
• delete by query
• delete by query rethrottle
• delete script
• get script
• open PIT
• PUT script
• scripts painless execute
• update by query
• update by query rethrottle
  APIs.

Source of info:

• https://www.elastic.co/guide/en/elasticsearch/reference/current/docs.html
• https://www.elastic.co/guide/en/elasticsearch/reference/current/script-apis.html
• https://www.elastic.co/guide/en/elasticsearch/painless/current/painless-execute-api.html

[abdonpijpelink]

LGTM. I've left a couple of suggestions. Feel free to ignore.

[github-actions]

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

API	Status	Request	Response
create	🟢	24/24	23/23
delete_by_query_rethrottle	🟢	1/1	1/1
delete_by_query	🟢	5/5	4/4
delete_script	🟢	8/8	8/8
delete	🟢	34/34	34/34
get_script	🟢	12/12	12/12
open_point_in_time	🟢	10/10	10/10
put_script	🟢	9/9	9/9
scripts_painless_execute	🟢	2/2	2/2
update_by_query_rethrottle	🟢	1/1	1/1
update_by_query	🟢	5/5	4/4

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