Stop using shortcuts in function_score YAML tests

[pquentin]

Those shortcuts are not supported by the Elasticsearch specification. This is a common source of issues for users:

• Cannot construct a desired FunctionScoreQuery elasticsearch-net#6060
• can't use script_score in function_score elasticsearch-java#989
• Function Score generates null elasticsearch-net#7410
• Decay functions missing elasticsearch-net#8119
• Creating nested function score query from json via supplied reader elasticsearch-java#917
• https://discuss.elastic.co/t/using-function-score-query-with-field-weighting-without-nest/369244
• https://discuss.elastic.co/t/constructing-a-functionscorequery-using-nest/290251
• https://discuss.elastic.co/t/using-function-score-query-with-field-weighting-without-nest/369244

I'm suggesting here that we stop documenting the "shortcut" version, where the function is inlined, and instead always recommend using "functions," which accepts a single object as well as an array.

What do you think?

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[elasticsearchmachine]

Pinging @elastic/es-search-foundations (Team:Search Foundations)

[elasticsearchmachine]

Pinging @elastic/core-docs (Team:Docs)

[pquentin]

Just realized that an array is actually mandatory here, I'll fix this PR. Sorry!

elasticsearch/server/src/main/java/org/elasticsearch/index/query/functionscore/FunctionScoreQueryBuilder.java

Lines 487 to 495 in 0a71ff4

	} else if (token == XContentParser.Token.START_ARRAY) {
	if (FUNCTIONS_FIELD.match(currentFieldName, parser.getDeprecationHandler())) {
	if (singleFunctionFound) {
	String errorString = "already found [" + singleFunctionName + "], now encountering [functions].";
	handleMisplacedFunctionsDeclaration(parser.getTokenLocation(), errorString);
	}
	functionArrayFound = true;
	currentFieldName = parseFiltersAndFunctions(parser, filterFunctionBuilders);
	} else {

[pquentin]

Closing for now, I may reopen in the future by using the full form:

GET /_search
{
  "query": {
    "function_score": {
      "query": { "match_all": {} },
      "boost": "5",
      "functions": [
        {
          "filter": { "match": { "test": "bar" } },
          "random_score": {},
          "weight": 23
        }
      ],
     }
  }
}
```