docs: add graphql multi-field sort note (#15123)

### What?
Adding a note to the sorting docs that you can sort on multiple fields
in the GraphQL endpoint.

### Why?
I was confused about how to do this in the GraphQL API because you can't
pass in an array there. Adding a specific note will help future
travelers.

**Edit:**
I just reread the docs and see that it's there under the REST API
heading. I feel the note still adds something.

Author: atimmer

docs/queries/sort.mdx

@@ -101,3 +101,15 @@ query {
   }
 }
 ```
+
+To sort by multiple fields in GraphQL, pass a comma-separated list:
+
+```ts
+query {
+  Posts(sort: "priority,-createdAt") {
+    docs {
+      color
+    }
+  }
+}
+```