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>

man/src/login.md

@@ -19,4 +19,4 @@ credentials.  When you log in, the command stores encoded credentials in
     # docker login localhost:8080
 
 # See also
-**docker-logout(1)** to log out from a Docker registry.
+**docker-logout(1)** to log out from a registry.

man/src/logout.md

@@ -10,4 +10,4 @@ located at `https://registry-1.docker.io/` by default.
     # docker logout localhost:8080
 
 # See also
-**docker-login(1)** to log in to a Docker registry server.
+**docker-login(1)** to log in to a registry.