[DOCS] Add operation summaries for data stream APIs

[shainaraskas]

This PR adds operation summaries for the data stream and data stream lifecycle APIs, using text from the appropriate pages from the elasticsearch API reference.

A couple of terminology fixes for data streams in the types area while I'm here as well

part of #2635

[github-actions]

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

API	Status	Request	Response
indices.add_block	🟢	2/2	2/2
indices.analyze	🟢	20/20	20/20
indices.clear_cache	🟢	4/4	4/4
indices.clone	🟢	5/5	5/5
indices.close	🟢	50/50	50/50
indices.create_data_stream	🟢	28/28	28/28
indices.create	🔴	916/957	957/957
indices.data_streams_stats	🟢	4/4	4/4
indices.delete_alias	🔴	15/15	5/15
indices.delete_data_lifecycle	⚪	Missing test	Missing test
indices.delete_data_stream	🟢	28/28	28/28
indices.delete_index_template	🟢	1/1	1/1
indices.delete_template	🟢	9/9	9/9
indices.delete	🟢	105/105	105/105
indices.disk_usage	🟢	5/5	5/5
indices.downsample	🟢	1/1	1/1
indices.exists_alias	🟢	38/38	38/38
indices.exists_index_template	⚪	Missing test	Missing test
indices.exists_template	🟢	15/15	15/15
indices.exists	🟢	39/39	39/39
indices.explain_data_lifecycle	⚪	Missing test	Missing test
indices.field_usage_stats	🟢	5/5	5/5
indices.flush	🟢	30/30	30/30
indices.forcemerge	🔴	12/12	11/12
indices.get_alias	🔴	86/86	74/86
indices.get_data_lifecycle	⚪	Missing test	Missing test
indices.get_data_stream	🔴	12/12	3/12
indices.get_field_mapping	🔴	15/15	14/15
indices.get_index_template	🔴	19/19	15/19
indices.get_mapping	🔴	121/121	109/121
indices.get_settings	🔴	61/61	59/61
indices.get_template	🟢	30/30	30/30
indices.get	🟢	53/53	53/53
indices.migrate_to_data_stream	⚪	Missing test	Missing test
indices.modify_data_stream	⚪	Missing test	Missing test
indices.open	🟢	18/18	18/18
indices.promote_data_stream	⚪	Missing test	Missing test
indices.put_alias	🔴	59/59	20/59
indices.put_data_lifecycle	⚪	Missing test	Missing test
indices.put_index_template	🔴	50/52	52/52
indices.put_mapping	🔴	75/103	103/103
indices.put_settings	🔴	54/57	57/57
indices.put_template	🔴	44/49	49/49
indices.recovery	🟢	9/9	9/9
indices.refresh	🟢	265/265	265/265
indices.reload_search_analyzers	🟢	2/2	2/2
indices.resolve_cluster	🔴	10/12	12/12
indices.resolve_index	🟢	6/6	6/6
indices.rollover	🔴	26/26	1/26
indices.segments	🔴	6/6	5/6
indices.shard_stores	🔴	5/5	4/5
indices.shrink	🟢	5/5	5/5
indices.simulate_index_template	🔴	6/7	5/7
indices.simulate_template	🔴	5/5	4/5
indices.split	🟢	11/11	11/11
indices.stats	🔴	103/103	31/102
indices.unfreeze	🟢	1/1	1/1
indices.update_aliases	🔴	26/26	7/26
indices.validate_query	🟢	7/7	7/7

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

[lcawl]

I've added two musings but otherwise docs LGTM

[lcawl]

• Creates a data stream.

Do we need this in the summary too? If it looks better with the repetition in the summary in description, I'm okay with it, just wondering whether we want to do that in general or not.

[shainaraskas]

I think that for the reference to look right, we generally want a description. them being identical is probably an indicator that something should be added to the description - I think a link to the data stream docs here, for example, would be super nice. I am avoiding messing with the descriptions too much in this PR, but I'd rather have the stub here so it's easier to improve them in bulk later. let me know what you think.

[lcawl]

• Converts an index alias to a data stream.

This is another one where I don't think repeating the summary adds a lot of value to the description.