tweak description of login/logout

Remove "Docker" from registry, as the registry specification is no
longer docker-specific, but part of the OCI distribution spec.

Also removed "Register" from one of the docs pages, as the login
command hasn't supported creating a new acccount on Docker Hub for
a long time.

I'm wondering if we should be more explicit about what log in / out
does (effectively; authenticate, and on success store the credentials
or token, and on log out; remove credentials/token).

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/dockerd.md

@@ -88,7 +88,7 @@ Options:
       --oom-score-adjust int                  Set the oom_score_adj for the daemon (default -500)
   -p, --pidfile string                        Path to use for daemon PID file (default "/var/run/docker.pid")
       --raw-logs                              Full timestamps without ANSI coloring
-      --registry-mirror list                  Preferred Docker registry mirror
+      --registry-mirror list                  Preferred registry mirror
       --rootless                              Enable rootless mode; typically used with RootlessKit
       --seccomp-profile string                Path to seccomp profile
       --selinux-enabled                       Enable selinux support