[DOCS] Document index.query.default_field index setting

[jrodewig]

Changes:

• Add docs for the index.query.default_field index setting to the Index modules page.
• Note in the MLT query docs that index.query.default_field is the default fields value. Other affected query types already mention the index.query.default_field index setting.

Closes #69770.

Previews

• Index modules docs: https://elasticsearch_69922.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/master/index-modules.html#index-max-regex-length
• MLT query docs: https://elasticsearch_69922.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/master/query-dsl-mlt-query.html#_parameters_2

[elasticmachine]

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

[elasticmachine]

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

[jtibshirani]

This looks good to me, thanks @jrodewig!

[jrodewig]

Thanks @jtibshirani!