Apply boost in script_score query to the whole query

[mayya-sharipova]

Current boost parameter in script_score query is wrongly only applied to the subquery.
For example, this query returns 1 without applying boost:

"query": {
    "script_score" : {
        "query" : {
            "match_all": {}
        },
        "script" : {
            "source" : "1"
        },
        "boost": 3
    }
}

While this query returns 3 applying boost to the subquery.

"query": {
    "script_score" : {
        "query" : {
            "match_all": {}
        },
        "script" : {
            "source" : "_score"
        },
        "boost": 3
    }
}

This behaviour was partially copied from function_score query where boost is applied to both subquery and the whole query. For example, this query returns 9 where boost is applied to the subquery and also the whole query (which is quite confusing).

"query": {
    "function_score" : {
        "query" : {
            "match_all": {}
        },
        "script_score" : {
            "script" : {
                "source" : "_score"
            }
        },
        "boost": 3
    }
}

We need to correct boost behaviour in script_score to apply only the whole query, and not subquery.

[elasticmachine]

Pinging @elastic/es-search (:Search/Ranking)

[mayya-sharipova]

Adding team-discuss option to understand the current boost behaviour in function_score query.

[mayya-sharipova]

We have decided to document this behaviour in function_score query. And correct the bug for script_score query.