minor #17480 Clarify how CollectionType entries are indexed (renepupil)

This PR was merged into the 5.4 branch.

Discussion
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Clarify how `CollectionType` entries are indexed

Clarifying the indexing for `CollectionType` entries prevents further confusion about how entries can be identified in the front end as well as when viewing the request data.

Commits
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81ae7bc Clarify how `CollectionType` entries are indexed

Author: OskarStark

reference/forms/types/collection.rst

@@ -11,6 +11,8 @@ forms, which is useful when creating forms that expose one-to-many
 relationships (e.g. a product from where you can manage many related product
 photos).
 
+When rendered, existing collection entries are indexed by the keys of the array that is passed as the collection type field data.
+
 +---------------------------+--------------------------------------------------------------------------+
 | Rendered as               | depends on the `entry_type`_ option                                      |
 +---------------------------+--------------------------------------------------------------------------+