diff --git a/README.md b/README.md
index 3f300b4..4c4d859 100644
--- a/README.md
+++ b/README.md
@@ -128,10 +128,13 @@ For the same image at quality 40 with fit `contain`, the input ends in
 Every setting can be given as an environment variable, in a YAML config
 file (`--config`), or both. A variable present in the environment wins over
 the file, even when it is empty, and the file wins over the built-in
-default. A variable's value is parsed as the same text in the file would
-be. The three lists take comma-separated entries, with the spaces around
-each trimmed; an empty variable is an empty list. A value that does not
-parse or is invalid aborts startup, naming the variable.
+default. The one exception is a variable named in the file's `env:` section:
+it is set while the file loads, so it overrides both the environment the
+process was started with and the file's own key. A variable's value is
+parsed as the same text in the file would be. The three lists take
+comma-separated entries, with the spaces around each trimmed; an empty
+variable is an empty list. A value that does not parse or is invalid aborts
+startup, naming the variable.
 
 | Variable                             | Config key                      | Meaning                                                                      |
 | ------------------------------------ | ------------------------------- | ---------------------------------------------------------------------------- |
diff --git a/config.example.yml b/config.example.yml
index 3edac9e..8df6a35 100644
--- a/config.example.yml
+++ b/config.example.yml
@@ -4,7 +4,10 @@
 # this file: PIXA_ plus the key in upper case, with "." written as "_"
 # (state_dir is PIXA_STATE_DIR, metrics.username is
 # PIXA_METRICS_USERNAME). The one exception is port, which is set by
-# PORT. In a variable, a list is comma-separated.
+# PORT. In a variable, a list is comma-separated. A variable named in
+# this file's env: section is set while the file loads, so it overrides
+# both the environment the process was started with and this file's own
+# key.
 
 # Server settings
 port: 8080