[DOCS] copy_to only works one level deep, not recursively (elastic#37249)

Author: geekpete

docs/reference/mapping/params/copy-to.asciidoc

@@ -62,3 +62,7 @@ Some important points:
 * It is the field _value_ which is copied, not the terms (which result from the analysis process).
 * The original <<mapping-source-field,`_source`>> field will not be modified to show the copied values.
 * The same value can be copied to multiple fields, with `"copy_to": [ "field_1", "field_2" ]`
+* You cannot copy recursively via intermediary fields such as a `copy_to` on 
+`field_1` to `field_2` and `copy_to` on `field_2` to `field_3` expecting 
+indexing into `field_1` will eventuate in `field_3`, instead use copy_to 
+directly to multiple fields from the originating field.