Add updater docs for #3843 (#3851)

Author: JoviDeCroock

docs/api/graphcache.md

@@ -106,6 +106,11 @@ given mutation field or subscription field is written to the cache. These side-e
 to update data in the cache that is implicitly changed on the GraphQL API, that _Graphcache_ can't
 know about automatically.
 
+For mutation fields that don't have an updater, Graphcache has a fallback: if a returned entity
+isn't currently found in the cache, it assumes a create-mutation and invalidates cached
+entities of that type. This behavior was introduced in Graphcache v7 and is skipped once an updater
+for the mutation field is added.
+
 ```ts
 interface UpdatesConfig {
   Mutation: {

docs/graphcache/cache-updates.md

@@ -59,6 +59,18 @@ cacheExchange({
 });
 ```
 
+## Default mutation invalidation
+
+Starting in [Graphcache v7](https://github.com/urql-graphql/urql/blob/main/exchanges/graphcache/CHANGELOG.md#700),
+mutations without a configured `updates.Mutation.<fieldName>` updater have a fallback behavior:
+
+- If the mutation returns an entity that can't be found in the cache yet, Graphcache treats this as
+  a "create" mutation.
+- Graphcache then invalidates cached entities of the same `__typename` as the one returned from the mutation, which can trigger related queries to refetch.
+
+As soon as you define an updater for that mutation field, this fallback behavior no longer runs and
+your updater fully controls what happens after the mutation write.
+
 An "updater" may be attached to a `Mutation` or `Subscription` field and accepts four positional
 arguments, which are the same as [the resolvers' arguments](./local-resolvers.md):