[DOCS] Adds description to global APIs part 3

[szabosteve]

Overview

Related to #964.

Adds descriptions to the:

• document exists
• source exists
• explain
• GET
• index
• msearch template
• reindex rethrottle
• render search template
• search shards
• search template
  APIs.

Source of info:

• https://www.elastic.co/guide/en/elasticsearch/reference/current/docs.html
• https://www.elastic.co/guide/en/elasticsearch/reference/current/search.html

[abdonpijpelink]

Great job on finishing these last remaining APIs 🚀. LGTM. I've left a few small suggestions.

[github-actions]

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

API	Status	Request	Response
exists_source	🟢	3/3	3/3
exists	🟢	19/19	19/19
explain	🟢	14/14	14/14
get	🟢	94/94	91/91
index	🟢	677/677	679/679
msearch_template	🟢	1/1	1/1
reindex_rethrottle	🟢	2/2	2/2
render_search_template	🟢	1/1	1/1
search_shards	🟢	8/8	8/8
search_template	🟢	2/2	1/1

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