We frequently see new issues mentioning that something defined
in `before_script` or `script` doesn't work with `after_script`.
Our documentation mentions that first two are concatenated and
executed together, why the third one is executed in a separated
shell context. But it looks like it's not clear enough.
Since this misunderstanding of how scripts are being executed
is repeated frequently, let's make it more clear in docs.
Tomasz Maczukin