More clarity and precisions on multi relations support (#2796) (#2799)

* Add more info and precisions on multi relations support

* Fix typos and mistake with Document Service API

Co-authored-by: meganelacheny <megane.lacheny@strapi.io>

Author: pwizla

docusaurus/docs/cms/api/rest/relations.md

@@ -17,14 +17,18 @@ Defining relations between content-types (that are designated as entities in the
 
 Relations between content-types can be managed through the [admin panel](/cms/features/content-manager#relational-fields) or through [REST API](/cms/api/rest) or [Document Service API](/cms/api/document-service) requests.
 
-Relations can be connected, disconnected or set through the Content API by passing parameters in the body of the request:
+Relations can be connected, disconnected or set through the Content API by passing parameters in the body of the request. These payloads work for both single-entry relations and multi relations (one-to-many, many-to-one, many-to-many, and many-way). When a relational field allows multiple links, the API expects arrays of relation IDs and returns arrays in responses.
 
 |  Parameter name         | Description | Type of update |
 |-------------------------|-------------|----------------|
 | [`connect`](#connect)   | Connects new entities.<br /><br />Can be used in combination with `disconnect`.<br /><br />Can be used with [positional arguments](#relations-reordering) to define an order for relations.    | Partial |
 | [`disconnect`](#disconnect)    | Disconnects entities.<br /><br />Can be used in combination with `connect`. | Partial |
 | [`set`](#set)           | Set entities to a specific set. Using `set` will overwrite all existing connections to other entities.<br /><br />Cannot be used in combination with `connect` or `disconnect`.  | Full |
 
+:::note
+Multi relations can be managed from the REST API and the [GraphQL API](/cms/api/graphql#fetch-relations): the  `connect`, `disconnect`, and `set` operations are available across both APIs. However, the [Document Service API](/cms/api/document-service) does not handle relations.
+:::
+
 :::note
 When [Internationalization (i18n)](/cms/features/internationalization) is enabled on the content-type, you can also pass a locale to set relations for a specific locale, as in this Document Service API example:
 

docusaurus/docs/cms/backend-customization/models.md

@@ -248,7 +248,9 @@ The `uid` type is used to automatically prefill the field value in the admin pan
 
 #### Relations
 
-Relations link content-types together. Relations are explicitly defined in the [attributes](#model-attributes)  of a model with `type: 'relation'`  and accept the following additional parameters:
+Relations link content-types together. Strapi supports both single-entry relations (one-way and one-to-one) and multi relations where at least one side can point to several entries (one-to-many, many-to-one, many-to-many, and many-way). Multi relations are persisted as arrays in the database layer and are returned as arrays in the Content API responses.
+
+Relations are explicitly defined in the [attributes](#model-attributes) of a model with `type: 'relation'` and accept the following additional parameters:
 
 | Parameter                         | Description                                                                                                                                     |
 | --------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------- |

docusaurus/docs/cms/features/content-type-builder.md

@@ -448,6 +448,10 @@ There are 6 different types of relations:
 - <img width="25" src="/img/assets/icons/v5/ctb_relation_manytomany.svg" /> Many-to-many: Content-type A *has and belongs to many* Content-type B
 - <img width="25" src="/img/assets/icons/v5/ctb_relation_manyway.svg" /> Many way: Content-type A *has many* Content-type B
 
+:::info Multi relations and single relations
+Relations where at least one side can reference several entries are called multi relations. In the Content-type Builder, this includes one-to-many, many-to-one, many-to-many, and many-way relations. These relations appear as multi-select fields in the Content Manager and return arrays from the REST, GraphQL, and Document Service APIs; while single relations (one-way and one-to-one relations) return a single linked entry (see [Managing relations with API requests](/cms/api/rest/relations) for more information).
+:::
+
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