Doc: /mnt is intended for temporary mounts only...

As far as I can see, according to the Filesystem Hierarchy Standard, `/mtn` is intended for temporary mounts only. See e.g. then man page `hier(7)`:

> /mnt This directory is a mount point for a temporarily mounted filesystem. In some distributions, /mnt contains subdirectories intended to be used as mount points for several temporary filesystems.

Or https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch03s12.html :

> This directory is provided so that the system administrator may temporarily mount a filesystem as needed. The content of this directory is a local issue and should not affect the manner in which any program is run.
>
> This directory must not be used by installation programs: a suitable temporary directory not in use by the system must be used instead.

The btrbk documentation currently rather encourages to permanently mount subvolumes there, e.g. in the README:

> /etc/fstab:
>
> `/dev/sda1 /mnt/btr_pool btrfs subvolid=5,noatime 0 0`

Now changing all the documentation would be tedious, and for demonstration and explaining how it works it's maybe good to use `/mnt` in the examples, but maybe the fstab example should be removed, or changed to `noauto`. Or a note somewhere about the FHS not recommending mounting filesystems permanently at `/mnt` would be useful.