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

www/apps/api-reference/specs/admin/paths/admin_customers.yaml

@@ -12,8 +12,11 @@ get:
         Comma-separated fields that should be included in the returned data. if
         a field is prefixed with `+` it will be added to the default fields,
         using `-` will remove it from the default fields. without prefix it will
-        replace the entire default fields. NOTE: This route doesn't allow
-        expanding custom relations.
+        replace the entire default fields. This API route restricts the fields
+        that can be selected. Learn how to override the retrievable fields in
+        the [Retrieve Custom
+        Links](https://docs.medusajs.com/learn/fundamentals/api-routes/retrieve-custom-links)
+        documentation.
       required: false
       schema:
         type: string
@@ -22,8 +25,11 @@ get:
           Comma-separated fields that should be included in the returned data.
           if a field is prefixed with `+` it will be added to the default
           fields, using `-` will remove it from the default fields. without
-          prefix it will replace the entire default fields. NOTE: This route
-          doesn't allow expanding custom relations.
+          prefix it will replace the entire default fields. This API route
+          restricts the fields that can be selected. Learn how to override the
+          retrievable fields in the [Retrieve Custom
+          Links](https://docs.medusajs.com/learn/fundamentals/api-routes/retrieve-custom-links)
+          documentation.
         externalDocs:
           url: '#select-fields-and-relations'
     - name: offset
@@ -4227,8 +4233,11 @@ post:
         Comma-separated fields that should be included in the returned data. if
         a field is prefixed with `+` it will be added to the default fields,
         using `-` will remove it from the default fields. without prefix it will
-        replace the entire default fields. NOTE: This route doesn't allow
-        expanding custom relations.
+        replace the entire default fields. This API route restricts the fields
+        that can be selected. Learn how to override the retrievable fields in
+        the [Retrieve Custom
+        Links](https://docs.medusajs.com/learn/fundamentals/api-routes/retrieve-custom-links)
+        documentation.
       required: false
       schema:
         type: string
@@ -4237,8 +4246,11 @@ post:
           Comma-separated fields that should be included in the returned data.
           if a field is prefixed with `+` it will be added to the default
           fields, using `-` will remove it from the default fields. without
-          prefix it will replace the entire default fields. NOTE: This route
-          doesn't allow expanding custom relations.
+          prefix it will replace the entire default fields. This API route
+          restricts the fields that can be selected. Learn how to override the
+          retrievable fields in the [Retrieve Custom
+          Links](https://docs.medusajs.com/learn/fundamentals/api-routes/retrieve-custom-links)
+          documentation.
         externalDocs:
           url: '#select-fields-and-relations'
   security: