documentation on expansion

Expansion in env attr is not consistent with most other rules

```
bats_test(
   name="test"
   srcs = [...]
   data = ["@terraform"]
   env = {
      "TERRAFORM_BIN" : "$(location @terraform)"
   }
)
```

`Error in fail: Unexpanded expression ('$(location @terraform)' in '$(location @terraform)').`


