login: slightly cleanup warning about unencrypted store

- Add an empty line before the warning to separate it from the command's output
- Use the `/go/` redirect URL that we have available.
- Put quotes around the filename used for storage.
- Use present tense for the message, as the message is printed while saving.
- User "credentials" instead of "password" for consistency with "credentials-store"

Before:

    docker login myregistry.example.com
    Username: thajeztah
    Password:
    WARNING! Your password will be stored unencrypted in /root/.docker/config.json.
    Configure a credential helper to remove this warning. See
    https://docs.docker.com/engine/reference/commandline/login/#credential-stores

    Login Succeeded

After:

    docker login myregistry.example.com
    Username: thajeztah
    Password:

    WARNING! Your credentials are stored unencrypted in '/root/.docker/config.json'.
    Configure a credential helper to remove this warning. See
    https://docs.docker.com/go/credential-store/

    Login Succeeded

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

cli/command/registry/login.go

@@ -23,9 +23,10 @@ import (
 // Otherwise, we'll assume they want it (sadly), because people may have been scripting
 // insecure logins and we don't want to break them. Maybe they'll see the warning in their
 // logs and fix things.
-const unencryptedWarning = `WARNING! Your password will be stored unencrypted in %s.
+const unencryptedWarning = `
+WARNING! Your credentials are stored unencrypted in '%s'.
 Configure a credential helper to remove this warning. See
-https://docs.docker.com/engine/reference/commandline/login/#credential-stores
+https://docs.docker.com/go/credential-store/
 `
 
 type loginOptions struct {