snapcraft export-login doesn't explain what the login file is

[Merlijn Sebrechts]

The help message of snapcraft export-login doesn't explain what the "login-file" input actually is.

Help message:

```
$ snapcraft export-login --help
Usage:
    snapcraft export-login [options] <<login-file>>

Summary:
    Log in to the Snap Store with your Ubuntu One SSO credentials.
    If you do not have any, you can create them at https://login.ubuntu.com

    To use the alternative authentication mechanism (Candid), set the
    environment variable 'SNAPCRAFT_STORE_AUTH' to 'candid'.

    This command exports credentials to use on systems where login is not
    possible or desired.

Options:
              -h, --help: Show this help message and exit
           -v, --verbose: Show debug information and be more verbose
             -q, --quiet: Only show warnings and errors, not progress
             --verbosity: Set the verbosity level to 'quiet', 'brief',
                           'verbose', 'debug' or 'trace'
           -V, --version: Show the application version and exit
                 --snaps: Comma-separated list of snaps to limit access
              --channels: Comma-separated list of channels to limit
                           access
                  --acls: Comma-separated list of ACLs to limit access
               --expires: Date/time (in ISO 8601) when this exported
                           login expires
    --experimental-login: Deprecated option to enable candid login. Set
                           SNAPCRAFT_STORE_AUTH=candid instead

See also:
    login
    logout
    whoami

For a summary of all commands, run 'snapcraft help --all'.
```

[Merlijn Sebrechts (merlijn-sebrechts)]

The docs also doesn't explain what file we need: https://snapcraft.io/docs/snapcraft-authentication