docs: clarify @tag repeatable requirements

docs/source/supergraphs.md

@@ -86,4 +86,21 @@ We aim to reduce the frequency at which these paired updates are necessary by ma
 |Rover version|Gateway version|
 |---|---|
 |<= v0.2.x|<= v0.38.x|
-|>= v0.3.x|>= v0.39.x|
+|>= v0.3.x|>= v0.39.x|
+
+##### `@tag` directives in newer versions of Rover/the gateway
+
+When updating to a gateway >= v0.39.x, all subgraph schemas with `@tag` directives will need to make those types repeatable not only on `FIELD_DEFINITION`, but also on `OBJECT`, `INTERFACE`, and `UNION`.
+
+Before:
+
+```graphql
+directive @tag(name: String!) repeatable on FIELD_DEFINITION
+```
+
+After:
+
+```graphql
+directive @tag(name: String!) repeatable on FIELD_DEFINITION | INTERFACE | OBJECT | UNION
+```
+