Clarify version compatibility in snapshot/restore docs (#39329)

Author: adoerrES

docs/reference/modules/snapshots.asciidoc

@@ -28,6 +28,10 @@ back up a cluster is by using the snapshot and restore functionality.
 [float]
 === Version compatibility
 
+IMPORTANT: Version compatibility refers to the underlying Lucene index
+compatibility. Follow the <<setup-upgrade,Upgrade documentation>>
+when migrating between versions.
+
 A snapshot contains a copy of the on-disk data structures that make up an
 index. This means that snapshots can only be restored to versions of
 Elasticsearch that can read the indices: