This PR is addressing the behavior described in #151.
The former version of `for` directive example usage ended in:
```
genqlient.graphql:<line>: for is only applicable to operations and arguments
```
I have:
- [x] Written a clear PR title and description (above)
- [x] Signed the [Khan Academy CLA](https://www.khanacademy.org/r/cla)
- [x] Added tests covering my changes, if applicable
- [x] Included a link to the issue fixed, if applicable
- [x] Included documentation, for new features
- [x] Added an entry to the changelog
Signed-off-by: Matúš Bafrnec <[email protected]>
