Automatic file update detection: better inform user how this works

### Steps to reproduce
1. Download a file using android client
2. Update&save it locally, e.g. add an annotation to a pdf
3. Verify that the file in the nextcloud dir (on the disk) is actually updated

### Expected behaviour
- Nextcloud should locate the change and update the file, uploading it into the cloud

### Actual behaviour
- Nextcloud doesn't reach on the update in any way. Reopening the file from the app forces the sync. Battery optimisation is disabled.

### Environment data
Android version: miui 10.3.2.0

Device model: mipad 4

Stock or customized system:

Nextcloud app version: 3.14.3

Nextcloud server version:

Reverse proxy: