Update configuration.md

content/en/hugo-modules/configuration.md

@@ -90,6 +90,12 @@ disable
 
 ## Module Config: mounts
 
+{{% note %}}
+When the `mounts` config was introduced in Hugo 0.56.0, we were careful to preserve the existing `staticDir` and similar configuration to make sure all existing sites just continued to work.
+
+But you should not have both. So you if you add a `mounts` section you should make it complete and remove the old `staticDir` etc. settings.
+{{% /note %}}
+
 {{< code-toggle file="config">}}
 [module]
 [[module.mounts]]