[docs] Clarify mount restrictions imposed by snap confinement

[ricab]

Description of documentation issue or suggestion

The documentation page for "mount" says

Because mounts are performed as privileged users (root on Linux and macOS, SYSTEM on Windows), they allow write access to the whole host operating system.

On Linux, this is only partially true: it does not hold if installed via snap.

Proposal to fix issue or enact suggestion

Explain that snap confinement prevents mounts outside of the /home directory (and to hidden files/folders in the /home directory) and possibly, removable media (depending on connected interfaces). Still, clarify that a user A with access to Multipass can still access mounts that a different user B was able to establish to B's home (i.e. outside of user's A home).

Additional context
https://snapcraft.io/docs/home-interface
https://snapcraft.io/docs/removable-media-interface