When creating a `/etc/systemd/system/docker.service`
unit file, that file _replaces_ the default
(`/lib/systemd/system/docker.service`) unit file.
While this may be intended by some, this section
describes how to use "partial" unit files ("drop-ins")
to override only certain properties of the default
unit file.
This patch removes the mention of
`/etc/systemd/system/docker.service` to prevent
users from inadvertently replacing the default
unit file.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>