Unify handling of swap space in docs and void-installer

[camoz]

Right now the official docs say in chroot.md for swap in the fstab:

UUID=1cb4[...]  swap        swap    rw,noatime,discard      0 0

fde.md says:

/dev/voidvm/swap  swap  swap    defaults              0       0

The void-installer does:

echo "UUID=$uuid none swap sw 0 0" >>$TARGET_FSTAB

Archwiki says:

UUID=device_UUID none swap defaults 0 0

fstab(5) says:

• for swap, use none for second field

If the discard option is used, then probably the docs + installer should also say/do swapon -d.