Fix two bugs in shells/fish

1. The build would fail when DOCS=off (PR 216167)
2. Tab completion doesn't work for "mount -t" (PR 216032)

PR: 216167
PR: 216032
Reported by: Maxfx <maficccc@gmail.com>
Reported by: David Guyot <david.guyot@europecamions-interactive.com>
Approved by: brd (ports)