docs: add backfills config to deploy guide

Author: chazmo03

Diff for: docs/docs/guides/deploy/dagster-yaml.md

@@ -142,6 +142,10 @@ retention:
       failure: 30
       success: -1
 
+backfills:
+  use_threads: true
+  num_workers: 8
+
 sensors:
   use_threads: true
   num_workers: 8
@@ -278,6 +282,16 @@ retention:
       success: -1
 ```
 
+### `backfills`
+
+Configures how backfills are evaluated.
+
+```yaml
+backfills:
+  use_threads: true
+  num_workers: 8
+```
+
 ### `sensors`
 
 Configures how sensors are evaluated.