Documents and data can be either documents and files you have uploaded to your iCloud Drive
iCloud Drive stores your documents—including your presentations, spreadsheets, PDFs, and images—in iCloud, so you can access them from any of your devices set up with iCloud. It allows your apps to share documents so you can work on the same file across multiple apps.
From: About iCloud Drive
Or it can be data from Apps that you have installed that use iCloud storage to backup data to keep it consistent across your devices.
Many apps automatically back up to iCloud after you install them. You can change which apps back up to iCloud and remove existing backups of that app's data from your storage.
From: Manage your iCloud storage
Its that second part of the quote that you should be able to do on the phone, but there is a software issue here that is preventing it from working correctly. If you want to keep troubleshooting that, we can start by quitting all the running apps on your phone:
Force an app to close in iOS
1. Press the Home button two times quickly. You'll see small previews of your recently used apps.
2. Swipe left to find the app you want to close.
3. Swipe up on the app's preview to close it.
When you have done that restart the device and test the issue again:
iOS: Turning off and on (restarting) and resetting
If that does get your Documents and Data to show in your settings I would next sign out of iCloud in your Settings:
On your iOS device: Go to Settings > iCloud, then at the bottom of the screen, tap Sign Out (Delete Account in iOS 7 and earlier).
From: iCloud: Change iCloud feature settings
Then quit the Settings app one more time and restart again and test it out. Let us know how that goes!