Cleanup union type docs

docs/reference/api/introduction.md

@@ -78,7 +78,7 @@ nullifying the field:
 }
 ```
 
-#### One-to-Many (/ Many-to-Many / Many-to-Any)
+#### One-to-Many (/ Many-to-Many)
 
 One-to-Many, and therefore Many-to-Many and Many-to-Any, relationships can be updated in one of two ways:
 
@@ -145,3 +145,70 @@ foreign key. This means that your data will never suddenly disappear, but it als
 orphaned items.
 
 :::
+
+#### Many-to-Any (Union Types)
+
+Many-to-Any fields work very similar to a "regular" many-to-many, with the exception that the related field can pull in
+the fields from any of the related collections, for example:
+
+```json
+{
+	"sections": [
+		{
+			"collection": "headings",
+			"item": {
+				/* headings fields */
+			}
+		},
+		{
+			"collection": "paragraphs",
+			"item": {
+				/* paragraphs fields */
+			}
+		}
+	]
+}
+```
+
+##### REST API
+
+To scope the fields that are returned per collection type, you can use the `<field>:<scope>` syntax in the fields
+parameter as follows:
+
+```
+GET /items/pages
+	?fields[]=sections.item:headings.id
+	&fields[]=sections.item:headings.title
+	&fields[]=sections.item:paragraphs.body
+	&fields[]=sections.item:paragraphs.background_color
+```
+
+##### GraphQL
+
+In GraphQL, you can use nested fragments on the Union Type to select the fields:
+
+```graphql
+query {
+	pages {
+		sections {
+			item {
+				... on headings {
+					id
+					title
+				}
+
+				... on paragraphs {
+					body
+					background_color
+				}
+			}
+		}
+	}
+}
+```
+
+::: tip Updating
+
+Updating records in a many-to-any is identical to the other relationship types.
+
+:::

docs/reference/api/items.md

@@ -74,10 +74,6 @@ will be an empty array.
 
 If your collection is a singleton, this endpoint will return the item.
 
-#### Example for M2M relation
-
-Let's imagine each article can be related to one or multiple `themes`.
-
 </div>
 <div class="right">
 
@@ -115,27 +111,6 @@ query {
 }
 ```
 
-##### Example with themes related to an article with a M2M relation
-
-```graphql
-query {
-	articles {
-		id
-		title
-		author {
-			first_name
-		}
-		themes {
-		    item {
-		      ... on them {
-			  label
-		      }
-		    }
-          	}
-	}
-}
-```
-
 </div>
 </div>