Clarify docs about ARG in FROM
@@ -530,8 +530,10 @@ FROM extras:${CODE_VERSION}
CMD /code/run-extras
```
-To use the default value of an `ARG` declared before the first `FROM` use an
-`ARG` instruction without a value:
+An `ARG` declared before a `FROM` is outside of a build stage, so it
+can't be used in any instruction after a `FROM`. To use the default value of
+an `ARG` declared before the first `FROM` use an `ARG` instruction without
+a value inside of a build stage:
```Dockerfile
ARG SETTINGS=default
