AA: add some short doc about autosync

docs/maintainers/AA/aa-archive-related-services.md

@@ -18,6 +18,38 @@ This page documents the services used by the
 
 ### Debian auto-import
 
+When the next version of Ubuntu is in full development, we will automatically
+pull some packages from Debian `unstable` into the new series' `-proposed`
+pocket. Importantly, we're only importing the sources, and rebuild the binaries
+from scratch.
+
+The service is set up on the `ubuntu-archive-toolbox` machine, as a cronjob
+running every 6 hours. The script itself is `auto-sync` in the
+[`ubuntu-archive-tool`
+repo](https://code.launchpad.net/~ubuntu-archive/ubuntu-archive-tools/+git/ubuntu-archive-tools/).
+
+Packages eligible for the sync are any package that doesn't have
+Ubuntu-specific changes. The heuristic is based on version number: if it
+contains `ubuntu`, it is considered to have a delta.
+
+In addition to delta-containing packages, some packages are entirely exluded from
+the sync through the [sync-blocklist](https://code.launchpad.net/~ubuntu-archive/+git/sync-blocklist).
+
+#### Disabling auto-import
+
+To disable auto-import, the usual procedure is to edit the
+`ubuntu-archive-toolbox` crontab to add a `--dry-run` parameter to the
+appropriate line, e.g.
+
+```cron
+# Before
+0 5,11,17,23 * * *      PYTHONPATH=/home/ubuntu-archive/python auto-sync --log-directory ~/public_html/auto-sync --batch
+# After
+0 5,11,17,23 * * *      PYTHONPATH=/home/ubuntu-archive/python auto-sync --dry-run --log-directory ~/public_html/auto-sync --batch
+```
+
+This allows for the continuing generation of logs, potentially useful for
+monitoring.
 
 ### `phased-updater`