Addressing comments

Signed-off-by: Nicolas Arnold <nic@roadie.io>

docs/auth/github/provider.md

@@ -41,6 +41,8 @@ auth:
         clientSecret: ${AUTH_GITHUB_CLIENT_SECRET}
         ## uncomment if using GitHub Enterprise
         # enterpriseInstanceUrl: ${AUTH_GITHUB_ENTERPRISE_INSTANCE_URL}
+        ## uncomment if intermediate service is used when using github apps
+        # callbackUrl: ${AUTH_GITHUB_CALLBACK_URL}
 ```
 
 The GitHub provider is a structure with three configuration keys:
@@ -50,6 +52,10 @@ The GitHub provider is a structure with three configuration keys:
 - `clientSecret`: The client secret tied to the generated client ID.
 - `enterpriseInstanceUrl` (optional): The base URL for a GitHub Enterprise
   instance, e.g. `https://ghe.<company>.com`. Only needed for GitHub Enterprise.
+- `callbackUrl` (optional): The callback url that GitHub will use when
+  initiating an OAuth flow, e.g.
+  `https://your-intermediate-service.com/handler`. Only needed if Backstage is
+  not the immediate receiver (e.g. one OAuth app for many backstage instances).
 
 ## Adding the provider to the Backstage frontend