Add docs for expiration

Author: cschleiden

docs/source/includes/_guide.md

@@ -617,12 +617,26 @@ if err != nil {
 
 ### Automatically expiring finished workflow instances
 
+This differs for different backend implementations.
+
+#### SQLite & MySQL
+
+```go
+client.StartAutoExpiration(ctx context.Context, delay time.Duration)
+```
+
+When you call this on a client, a workflow will be started in the `system` queue that will periodically run and remove finished workflow instances that are older than the specified delay.
+
+<div style="clear: both"></div>
+
+#### Redis
+
 ```go
 b, err := redis.NewRedisBackend(redisClient, redis.WithAutoExpiration(time.Hour * 48))
 // ...
 ```
 
-For now this is only supported for the Redis backend. When an `AutoExpiration` is passed to the backend, finished workflow instances will be automatically removed after the specified duration. This works by setting a TTL on the Redis keys for finished workflow instances. If `AutoExpiration` is set to `0` (the default), no TTL will be set.
+When an `AutoExpiration` is passed to the backend, finished workflow instances will be automatically removed after the specified duration. This works by setting a TTL on the Redis keys for finished workflow instances. If `AutoExpiration` is set to `0` (the default), no TTL will be set.
 
 ## Logging