Add note about fetchApi not being suitable for sign-in page use

Signed-off-by: Fredrik Adelöw <freben@gmail.com>

.changeset/sweet-apricots-kneel.md

@@ -0,0 +1,5 @@
+---
+'@backstage/core-plugin-api': patch
+---
+
+Add a note that the `fetchApi` utility should not be used on sign-in page implementations and similar.

packages/core-plugin-api/src/apis/definitions/FetchApi.ts

@@ -37,6 +37,13 @@ export type FetchApi = {
  * This is a wrapper for the fetch API, that has additional behaviors such as
  * the ability to automatically inject auth information where necessary.
  *
+ * Note that the default behavior of this API (unless overridden by your org),
+ * is to require that the user is already signed in so that it has auth
+ * information to inject. Therefore, using the default implementation of this
+ * utility API e.g. on the `SignInPage` or similar, would cause issues. In
+ * special circumstances like those, you can use the regular system `fetch`
+ * instead.
+ *
  * @public
  */
 export const fetchApiRef: ApiRef<FetchApi> = createApiRef({