documentation of posixfs command

[bjo81]

I recently migrated from Nextcloud by copying all data into the storage of my opencloud users and ran opencloud backup consistency -p /var/lib/opencloud/storage/users.

Now I stumbled upon opencloud-eu/opencloud#3182 and saw that a posixfs command exists which can be run with scan or consistency, and the latter can also fix inconsistencies.

But it seems the commands don't exist in the docs, they wasn't added in #398 or #1004 .