docs: clarify limitations for updating dependencies on non-default branches

Author: vaind

updater/README.md

@@ -84,6 +84,7 @@ jobs:
           api-token: ${{ secrets.CI_DEPLOY_KEY }}
 
   # Update dependencies on a non-default branch (e.g., alpha, beta, or version branches)
+  # Note: due to limitations in GitHub Actions' schedule trigger, this code needs to be pushed to the default branch.
   cocoa-v7:
     runs-on: ubuntu-latest
     steps:
@@ -155,4 +156,4 @@ If you're migrating from the v2 reusable workflow, see the [changelog migration
 Key changes:
 - Add `runs-on` to specify the runner
 - Move `secrets.api-token` to `with.api-token`
-- No need for explicit `actions/checkout` step (handled internally)
+- No need for explicit `actions/checkout` step (handled internally)