Instructions for upgrading the snap install are incomplete/misleading

[roerd]

The instructions for upgrading snaps (in package_upgrade.rst) are incomplete resp. misleading because they only the commands sudo snap refresh nextcloud without mentioning channels.

Your Nextcloud installation from snap will usually be from a "channel" tied to a certain major version, so you will need to snap switch to change the channel to another major version first, e.g. if you're currenly on version 17, use sudo snap switch --channel=18 nextcloud to switch to version 18.

Also, the follow-up command mentioned to complete the upgrade, currently mentioned as sudo -u www-data php occ upgrade, should be sudo nextcloud.occ upgrade instead on a modern snap installation instead.