diff --git a/docs/conf/writing.md b/docs/conf/writing.md
index e0eb72f828..48104699c7 100644
--- a/docs/conf/writing.md
+++ b/docs/conf/writing.md
@@ -101,6 +101,9 @@ All loaded configuration files are merged together using the following rules:
   contents.
 - Objects are merged together deeply, meaning that if any of the included
   configs contain a value for a given path, it will be found.
+- A `null` value in a config file will be treated as an explicit absence of
+  configuration. This means that the reading will not fall back to a lower priority
+  config, but it will still be treated as if the configuration was not present.
 
 The priority of the configurations is determined by the following rules, in
 order: