docs: Document remote-ls without REMOTE

The "flatpak remote-ls" command can be used without specifying a remote,
so change the man page and --help output to reflect that.

Closes: #1210
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f