Document scaling down app on upgrade for snapshot

This commit documents change how chart devs can have certain workloads to be scaled down before taking a snapshot of the application's volumes on an upgrade of the application to a newer catalog item version.

docs/upgrade.md

+# Upgrading Installed Catalog Item
+
+TrueNAS Scale allows upgrading installed catalog item to a new available
+item version. TrueNAS Scale takes a snapshot of the existing PVC's / iX Volumes
+before an upgrade which actually allows the user to rollback easily to a previous
+catalog item version for any reason ( like some missing deprecated functionality or bug in the newer version ).
+
+When TrueNAS Scale takes a snapshot of the application's data ( this excludes explicit host path volumes ),
+TrueNAS does not stop any workloads before taking a snapshot as each workload
+might have its own custom upgrade strategy like Rolling Update and in this
+case TrueNAS should not be stopping the workload as that would violate the
+custom upgrade strategy. This works with most use cases, however there are
+workloads which should be stopped before a snapshot is taken of their data like
+databases. It's possible that if a database workload is not stopped before
+taking a snapshot, on a rollback it might get into an inconsistent state as
+it might have had active transactions at the time the snapshot was taken and rolling
+back to that period of time can result in inconsistent / undesired behavior
+from the database.
+
+For such custom scenarios, TrueNAS Scale allows to stop workloads specifically
+before a snapshot is taken of an application's volumes. Workloads in this case can
+add the following annotation to the deployment/statefulset:
+
+```
+"ix.upgrade.scale.down.workload": true
+```
+
+Deployment(s) / statefulset(s) having the above annotation set will be stopped
+by TrueNAS Scale before a snapshot is taken of the application's volume.