man: mention that x-systemd.device-timeout is only for fstab

Reformat fstab options description. Now they are easier to read and
show up in systemd.directives(7).

Use a single sublist for both /etc/fstab and /etc/crypttab options.
Many of them can be used in both places. crypttab(5) is updated to use
the same docbook elements, so formatting is uniform.