Merge pull request #11556 from robin13/master

Docs: More information about 'Copy field to'

docs/reference/mapping/types/core-types.asciidoc

@@ -563,7 +563,7 @@ The following Similarities are configured out-of-box:
 
 Adding `copy_to` parameter to any field mapping will cause all values of this field to be copied to fields specified in
 the parameter. In the following example all values from fields `title` and `abstract` will be copied to the field
-`meta_data`.
+`meta_data`. The field which is being copied to will be indexed (i.e. searchable, and available through `fielddata_field`) but the original source will not be modified.
 
 
 [source,js]