Document "scratch" mountpoint

[cirocosta]

Hey,

In https://github.com/concourse/concourse/issues/1045, there was the initial work to have /scratch, a mountpoint that allows one to bypass overlay and have a direct mount to the underlying filesystem (that overlay sits on top of).

While that detail is leveraged by resources like docker-image-resource, that is not documented in ci.concourse-ci.org.

Having this mount point documented under our docs could then allow folks to more easily discover this future and work around problems that overlay might cause once we migrate from btrfs as a default preference in detect to overlay (see https://github.com/concourse/concourse/issues/4071).

Thanks!