Improve description of keep_artifacts flag

deployment/README.md

@@ -55,7 +55,7 @@ These builds are not necessarily run from the `build` script, but are present fo
 
 * The `clean` action should always `exit 0` even if no work is done or it fails.
 
-* The `clean` action can be passed a variable as argument 1, named `keep_artifacts`. It is indended to handle situations when the user runs `build --keep-artifacts` and should be handled intelligently. Usually, this is used to preserve Docker images while still removing temporary directories.
+* The `clean` action can be passed a variable as argument 1, named `keep_artifacts`, containing either the value `y` or `n`. It is indended to handle situations when the user runs `build --keep-artifacts` and should be handled intelligently. Usually, this is used to preserve Docker images while still removing temporary directories.
 
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