Add summaries for root operations

[shainaraskas]

This PR adds operation summaries for operations at the root /, using text from the appropriate pages from the elasticsearch API reference.

part of #2635

will regenerate the reference after #2718 + #2719 is approved and merged and this PR is rebased

note for reviewers: most of these operations will probably eventually make it out of root and into their respective categories (mostly index/doc related)

[github-actions]

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

API	Status	Request	Response
cluster.info	🔴	4/4	2/4
cluster.reroute	🟢	6/6	5/5
create	🟢	30/30	29/29
delete	🟢	37/37	37/37
exists_source	🟢	3/3	3/3
exists	🟢	19/19	19/19
explain	🟢	14/14	14/14
get_source	🟢	18/18	18/18
get	🔴	268/283	278/280
index	🔴	1009/1025	1027/1027
indices.add_block	🟢	2/2	2/2
indices.create	🔴	920/957	957/957
indices.delete_alias	🔴	15/15	5/15
indices.delete	🟢	105/105	105/105
indices.exists_alias	🟢	38/38	38/38
indices.exists	🟢	39/39	39/39
indices.get_alias	🔴	86/86	74/86
indices.get_settings	🔴	61/61	59/61
indices.get	🟢	53/53	53/53
indices.put_alias	🔴	59/59	20/59
indices.rollover	🔴	26/26	1/26
indices.update_aliases	🔴	26/26	7/26
info	🟢	5/5	5/5
ping	🟢	1/1	1/1
search_mvt	🔴	34/34	0/34
termvectors	🟢	7/7	7/7
update	🟢	36/36	36/36

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

[lcawl]

One typo otherwise LGTM

[github-actions]

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

API	Status	Request	Response
cluster.info	🔴	4/4	2/4
create	🟢	30/30	29/29
delete	🟢	37/37	37/37
exists_source	🟢	3/3	3/3
exists	🟢	19/19	19/19
explain	🟢	14/14	14/14
get_source	🟢	18/18	18/18
get	🔴	268/283	278/280
index	🔴	1051/1067	1069/1069
indices.add_block	🟢	2/2	2/2
indices.create	🔴	933/970	970/970
indices.delete_alias	🔴	15/15	5/15
indices.delete	🟢	105/105	105/105
indices.exists	🟢	39/39	39/39
indices.get	🟢	53/53	53/53
info	🟢	5/5	5/5
ping	🟢	1/1	1/1
search_mvt	🔴	34/34	0/34
termvectors	🟢	7/7	7/7
update	🟢	36/36	36/36

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
cluster.info	🔴	4/4	2/4
create	🟢	30/30	29/29
delete	🟢	37/37	37/37
exists_source	🟢	3/3	3/3
exists	🟢	19/19	19/19
explain	🟢	14/14	14/14
get_source	🟢	18/18	18/18
get	🔴	268/283	278/280
index	🔴	1051/1067	1069/1069
indices.add_block	🟢	2/2	2/2
indices.create	🔴	933/970	970/970
indices.delete_alias	🔴	15/15	5/15
indices.delete	🟢	105/105	105/105
indices.exists	🟢	39/39	39/39
indices.get	🟢	53/53	53/53
info	🟢	5/5	5/5
ping	🟢	1/1	1/1
search_mvt	🔴	34/34	0/34
termvectors	🟢	7/7	7/7
update	🟢	36/36	36/36

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