Add a clarifying comment about deleted volumes (#8328)

During debugging, we saw a region snapshot replacement that was
operating on a deleted snapshot. The fact that this was ok was not
clear, so this commit adds a comment explaining why operating on a
snapshot with a hard-deleted volume is ok, and in fact required.

Author: jmpesp

nexus/src/app/background/tasks/region_snapshot_replacement_start.rs

@@ -246,6 +246,13 @@ impl RegionSnapshotReplacementDetector {
                 continue;
             }
 
+            // Do not check if the volume is deleted here: if the read-only
+            // target is _not_ deleted (checked above), then even if the
+            // snapshot volume is hard deleted there are still other references
+            // in other volumes to those read-only targets that need replacing.
+            // The start saga can handle if the snapshot volume is hard deleted,
+            // so proceed with invoking it here.
+
             let result = self
                 .send_start_request(
                     authn::saga::Serialized::for_opctx(opctx),