docs(self-hosted configuration): `globalExtends` can't be private (#21597)

Co-authored-by: Rhys Arkins <rhys@arkins.net>

docs/usage/self-hosted-configuration.md

@@ -470,6 +470,12 @@ Use the `globalExtends` field if your preset has any global-only configuration o
 
 Use the `extends` field instead of this if, for example, you need the ability for a repository config (e.g. `renovate.json`) to be able to use `ignorePresets` for any preset defined in global config.
 
+<!-- prettier-ignore -->
+!!! warning
+    `globalExtends` presets can't be private.
+    When Renovate resolves `globalExtends` it does not fully process the configuration.
+    This means that Renovate does not have the authentication it needs to fetch private things.
+
 ## logContext
 
 `logContext` is included with each log entry only if `logFormat="json"` - it is not included in the pretty log output.