dos: add docs on overriding allowed fields in routes (#12233)

www/apps/api-reference/markdown/admin.mdx

@@ -639,6 +639,35 @@ This returns the variants of each product, but the variants only have their `id`
 
 </DividedMarkdownLayout>
 
+<DividedMarkdownLayout>
+
+<DividedMarkdownContent>
+
+### Select Custom Linked Data Models
+
+Most of the API routes that accept a `fields` query parameter allow you to specify custom linked data models. For example, if you linked a `Brand` to a `Product`, you can pass `brand` in the `fields` query parameter to retrieve the brand of each product.
+
+However, some API routes restrict the fields and relations you can retrieve. To learn about those API routes and how to override the allowed fields and relations, refer to the [Retrieve Custom Linked Data Models from Medusa's API routes](!docs!/learn/fundamentals/api-routes/retrieve-custom-links) documentation.
+
+</DividedMarkdownContent>
+
+</DividedMarkdownLayout>
+
+<DividedMarkdownLayout>
+
+<Feedback
+  event="survey_api-ref"
+  extraData={{
+    area: "admin",
+    section: "select-fields"
+  }}
+  pathName="/api/admin"
+  question="Was this section helpful?"
+  vertical={true}
+/>
+
+</DividedMarkdownLayout>
+
 </SectionContainer>
 
 <SectionContainer noTopPadding={true}>