retain_visibility configuration reference clarification

Updated retain_visibility to allow null value for visibility checks.

Author: rvanlaak

docs/B-configuration-reference.md

@@ -52,7 +52,7 @@ flysystem:
                         private: 0o700
             visibility: ~ # default null. Possible values are 'public' or 'private'
             directory_visibility: ~ # default null. Possible values are 'public' or 'private'
-            retain_visibility: true # default true, will then lead to adapters performing visibility checks (e.g. GetObjectAcl command for S3 adapters)
+            retain_visibility: ~ # default null. When set to `true` or `null`, it will lead to adapters performing visibility checks (e.g. GetObjectAcl command for S3 adapters), hence `false` prevents retrieving an object's current visibility
             case_sensitive: true
             disable_asserts: false