[DOCS] Doc improvements for query rewrite parameter

[immavalls]

Elasticsearch version (bin/elasticsearch --version): 7.x

Plugins installed: []

JVM version (java -version): N/A

OS version (uname -a if on a Unix-like system): N/A

Description of the problem including expected versus actual behavior:

In version 6.8 the fuzzy query builder sets the rewrite method to constant_score if the clause is in a filter context and the option has not been set explicitly. This heuristic was lost when refactoring in 7.x but our docs still reflect constant_score as the default.

• https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-multi-term-rewrite.html#rewrite-param-valid-values

This was a finding by @jimczi and @cbuescher when troubleshooting an upgrade case, and they asked to open an issue to keep track of the doc improvement needed.

[elasticmachine]

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

[cbuescher]

Looking at the code a bit more, the prefix, wildcard and regex queries seem to default to constant_score rewrites in absence of a specific rewrite method, while fuzzy and using the fuzzy option in query_string seem to use top_terms_blended_freqs_N via Lucenes FuzzyQuery ctor.
I'm wondering if we should change the docs or if we should consider changing the default to constant_score everywhere. For example, it seems we expect QueryStringQueryBuilder to default to constant_score in absence of a query parameter here but that doesn't get used in all cases currently (e.g. QueryStringQueryParser#getFuzzyQuerySingle doesn't seem to re-set the rewrite methode while QueryStringQueryParser#newFuzzyQuery does).

[elasticmachine]

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