Documentation says all special filesystem tokens can be followed by a path

[smcv]

As @A6GibKm noted on flatpak/flatpak#3784, https://docs.flatpak.org/en/latest/sandbox-permissions-reference.html?highlight=filesystem claims that "Paths can be added to all the above filesystem options". They can't:

• host is allowed, but host/opt doesn't give you /opt, and probably shouldn't (although /opt does)
• home is allowed, but home/projects doesn't give you /home/you/projects (although ~/projects does)

The new host-etc and host-os (subsets of host), which aren't documented here yet, don't accept trailing paths either, and I think they probably shouldn't: they're special-cased and do not have the same effect as specifying /etc and /usr separately.

When I added host-etc and host-os to Flatpak, I updated flatpak-metadata(5) to provide a more formal specification of which filesystem strings are allowed and supported.