APIs that support flat settings are not documented

APIs that support flat settings are not documented. I would expect that somewhere support for flat settings is documented for update APIs. One example: ```json PUT _index_template/debug-template { "composed_of": [], "index_patterns": [ "yyyyyy-*" ], "template": { "mappings": { "properties": [] }, "settings": { "index.number_of_shards": 1, "index.codec": "default", "index.mode": "standard", "index.routing_partition_size": 1, "index.soft_deletes.retention_lease.period": "12h", "index.load_fixed_bitset_filters_eagerly": true, "index.shard.check_on_startup": "false", "index.number_of_replicas": 1, "index.search.idle.after": "30s", "index.refresh_interval": "10s", "index.max_result_window": 10000, "index.max_inner_result_window": 100, "index.max_rescore_window": 10000, "index.max_docvalue_fields_search": 100, "index.max_script_fields": 32, "index.max_ngram_diff": 1, "index.max_shingle_diff": 3, "index.analyze.max_token_count": 10000, "index.highlight.max_analyzed_offset": 1000000, "index.max_terms_count": 65536, "index.max_regex_length": 1000, "index.query.default_field": [ "*" ], "index.routing.allocation.enable": "all", "index.routing.rebalance.enable": "all", "index.gc_deletes": "60s", "index.default_pipeline": "_none", "index.final_pipeline": "_none", "index.hidden": false } }, "version": 1739874433 } ```