gitlab-based_packaging_workflow: Explain when a manual sync may be needed

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

content/guidelines/gitlab-based_packaging_workflow.md

@@ -52,7 +52,21 @@ either boot the Apertis [SDK]( {{< ref "/sdk.md" >}} ) or enter the
     docker pull docker-registry.apertis.org/apertis/apertis-${APERTIS_RELEASE}-package-source-builder
     docker run -it --rm --env HOME -w "$(pwd)" -v "$HOME:$HOME" -v "$HOME:/root" --security-opt label=disable apertis-${APERTIS_RELEASE}-package-source-builder bash
 
-## How to sync an Apertis package with a new version from Debian
+## How to manually sync an Apertis package with a new version
+
+Upstream updates are usually handled automatically by the
+[`ci-update-from-upstream.yml`](https://gitlab.apertis.org/infrastructure/ci-package-builder/)
+Continous Integration pipeline, which fetches upstream packages, merges them
+with the Apertis contents and directly creates Merge Requests to be reviewed
+by maintainers.
+
+However, in some cases it is necessary to manuall pull upstream contents:
+1. the CI failed to merge the upstream update with the downstream changes, so a
+   developer must reproduce the merge and fix the conflicts
+1. for some reason it in necessary to manually pull updates from a new upstream
+   distribution/suite
+
+The steps below can guide you on manually pulling upstream updates:
 
 1.  On GitLab, [fork the
     project](https://docs.gitlab.com/ce/user/project/repository/forking_workflow.html)