Fix fsync with --app=bundleId

[zemanj]

Fixes #593.

According to libimobiledevice issue 193 from 2015 (in particular, this comment), the service com.apple.mobile.house_arrest doesn't support the VendContainer command anymore (unless the app was installed via XCode).
However, for apps that have UIFileSharingEnabled set to true in their information property list, the VendDocuments command can be used instead to access the app's /Documents directory.

This PR replaces VendContainer by VendDocuments to make this work.

Note that when using the go-iOS fsync command with the option --app=BUNDLE_ID set, the PATH passed with the option --path=PATH has to start with /Documents (in particular, / won't work).
For example, to use the fsync tree command for a specific app, one has to call ios fsync --app=BUNDLE_ID tree --path=/Documents.