Update ignore_missing_submod option description

tanzaku · tanzaku · commit 50e7197e7851 · 2024-01-30T00:51:02.000+09:00
diff --git a/Configurations.md b/Configurations.md
@@ -1309,6 +1309,7 @@ ignore = ["/"]
 ## `ignore_missing_submod`
 
 Ignore missing submodule error.
+By default, missing modules will lead to module not found errors and rustfmt won't format anything.
 
 - **Default value**: `false`
 - **Possible values**: `true`, `false`
