Clarify default behavior of auto_create_index (#46134)

Be specific about the default behaviour of `action.auto_create_index` when a list is given.

Author: PhaedrusTheGreek

docs/reference/docs/index_.asciidoc

@@ -138,7 +138,8 @@ automatically. You can modify this setting to explicitly allow or block
 automatic creation of indices that match specified patterns, or set it to
 `false` to disable automatic index creation entirely. Specify a
 comma-separated list of patterns you want to allow, or prefix each pattern with
-`+` or `-` to indicate whether it should be allowed or blocked.
+`+` or `-` to indicate whether it should be allowed or blocked.  When a list is
+specified, the default behaviour is to disallow.
 
 [source,js]
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