Add descriptions for Search API

[abdonpijpelink]

Adds descriptions for the Search API. Source for the descriptions: https://www.elastic.co/guide/en/elasticsearch/reference/master/search-search.html (and pages linked from it)

Does not include aggs and suggesters.

[abdonpijpelink]

I'm not sure what to do about the validation error (if anything). Running make validate api=search stack-version=8.8.0 locally just returns:

✖ 
    at run (/Users/abdon/projects/github/elasticsearch-specification/compiler/run-validations.js:175:12)
    exit code: 1
make: *** [validate] Error 1

[lcawl]

I'm not sure what to do about the validation error (if anything).

I think the validation messages can be ignored in the context of adding descriptions. My interpretation of them is that they're indicating there are some request and response properties missing from the spec, but that's not what we're addressing with these PRs, unless we happen to notice an obvious omission.

[lcawl]

One suggestion, otherwise LGTM

[github-actions]

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

API	Status	Request	Response
search	🔴	1519/1538	1519/1520

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