Esta opção parece ter sido descartada. O manpage do Ubuntu para fstab
contém este texto se você selecionar 14.04 LTS no topo:
The mountall(8) program that mounts filesystem during boot also
recognises additional options that the ordinary mount(8) tool does not.
These are: ''bootwait'' which can be applied to remote filesystems
mounted outside of /usr or /var, without which mountall(8) would not
hold up the boot for these; ''nobootwait'' which can be applied to non-
remote filesystems to explicitly instruct mountall(8) not to hold up
the boot for them; ''optional'' which causes the entry to be ignored if
the filesystem type is not known at boot time; and ''showthrough''
which permits a mountpoint to be mounted before its parent mountpoint
(this latter should be used carefully, as it can cause boot hangs).
Este parágrafo não existe na versão da página que você recebe se você selecionar 16.04 na parte superior.