[Docs] Clarify mapping `index` option default (#27104)

docs/reference/mapping/params/index.asciidoc

@@ -2,5 +2,5 @@
 === `index`
 
 The `index` option controls whether field values are indexed. It accepts `true`
-or `false`. Fields that are not indexed are not queryable.
+or `false` and defaults to `true`. Fields that are not indexed are not queryable.