Add specific language to context docs that reference resolvers are necessary for contextual fields

docs/source/entities/use-contexts.mdx

@@ -191,3 +191,35 @@ type Transaction @key(fields: "id") {
   ): Int!
 }
 ``` 
+
+## A note on resolvers
+
+Fields that are used with the context directives must be resolvable via a [reference resolver](/federation/entities/#2-define-a-reference-resolver). Query plans that fetch contextual fields are based on the entity where the `@context` directive is set. Therefore, all subgraphs that resolve that entity must support looking up their fields based on the entity's `@keys` fields.
+
+Take the [previous example](#referencing-fields-across-subgraphs). Suppose the first subgraph has a single query to retrieve the logged in user:
+
+```graphql disableCopy showLineNumbers=false
+type Query {
+  me: User!
+} 
+```
+
+The following example resolver would not correctly support contextual variables:
+
+```typescript title="❌ Incorrect example" disableCopy showLineNumbers=false
+Query: {
+  me: () => ({
+    id: 1, userCurrency: { id: 431, currencyCode: 'USD'}
+  })
+}
+```
+
+This is because the fetch set to subgraph 1 to retrieve the `isoCode` would be done on the entity. Therefore a section to resolve a User by keys would need to be added:
+
+```typescript
+User: () => {
+  __resolveReference(partialUser) {
+    return fetchUserById(partialUser.id);
+  }
+}
+```