Say that a config file's env: section overrides the environment (closes #128)

A variable named in the config file's env: section is set while the
file loads, so it overrides both the environment the process was started
with and the file's own key. README.md and the config.example.yml header
said only that a variable wins over the file; they now state this
exception where the precedence is given.

Model: opus-5-5

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                                                                      |
 | ------------------------------------ | ------------------------------- | ---------------------------------------------------------------------------- |