amend: update the explanation in step one

docs/docs/preprocessing-external-images.md

@@ -85,7 +85,7 @@ exports.onCreateNode = async ({
 
 Going step by step through the code:
 
-1. Define some types for `MarkdownRemark` using the Schema Customization API. Defining a field for alternative text as `featuredImgAlt` can also improve accessibility, in addition to providing context for the image if it fails to load.
+1. Define some types for MarkdownRemark using the Schema Customization API. For `featuredImg`, use the `from` argument to point the `link` extension to the correct field name(with a `___NODE` suffix), [more details about foreign-key fields here](https://www.gatsbyjs.org/docs/schema-customization/#foreign-key-fields). Defining a field for alternative text as `featuredImgAlt` can also improve accessibility, in addition to providing context for the image if it fails to load.
 2. Create an `onCreateNode` function so you can watch for when `MarkdownRemark` nodes are made.
 3. Use `createRemoteFileNode` by passing in the various required fields and get a reference to the file afterwards.
 4. If the Node is created, attach it as a child of the original Node. `___NODE` tells the GraphQL layer that the name before it is going to be a field on the parent Node that links to another Node. To do this, pass the `id` as the reference. Do note, this new node is now attached to the root of the `markdownRemark` node instead of the `frontmatter` field.