nixos/nextcloud: add basic module documentation and warn about current upgrading issues

Part of #49783. NextCloud tracks in its `config.php` the application's
state which makes it hard for the module to modify configurations during
upgrades.

It will take time until the issue is properly fixed, therefore we
decided to warn about this in the manual.

This PR addresses two things:

* Adding a basic example for nextcloud. I figured it to be helpful to
  add some basic usage instructions when adding a new manual entry.
  Advanced documentation may follow later.

  For now this document actively links to the service options, so users
  are guided to the remaining options that can be helpful in certain
  cases.

* Add a warning about upgrades and manual changes in
  `/var/lib/nextcloud`. This will be fixed in the future, but it's
  definetely helpful to document the current issues in the manual (as
  proposed in https://github.com/NixOS/nixpkgs/issues/49783#issuecomment-439691127).