Breaking changes note for frozen and MDP

Relates elastic#71896

docs/reference/migration/migrate_7_13.asciidoc

@@ -60,6 +60,23 @@ comma-delimited string. These are now passed one-by-one as single
 geohash strings.
 ====
 
+[discrete]
+[[breaking_713_frozen_multiple_data_paths_changes]]
+==== Frozen tier and multiple data paths changes
+
+{es} 7.12 included a technical preview of the frozen tier, being able to use
+partially mounted indices (searchable snapshots mounted with the shared cache
+option). Trying out this feature required configuring a shared cache using the
+`xpack.searchable.snapshot.shared_cache.size` setting.
+
+In {es} 7.13+, having a non-zero `xpack.searchable.snapshot.shared_cache.size`
+on nodes utilizing multiple data paths (`path.data` points to multiple
+locations) is no longer supported and will prevent the node from starting. If
+you do not utilize multiple data paths this will not affect you. Likewise, if
+you have not set `xpack.searchable.snapshot.shared_cache.size` and have not
+configured dedicated frozen nodes (nodes with the `data_frozen` role and no
+other data roles) this will not affect you.
+
 [discrete]
 [[deprecated-7.13]]
 === Deprecations