Doc ability to use `--` in lieu of current repo.

`browse` command will act buggy for subpages without this.

Solves #719 
Good start, but this should be:

```
[ [[<USER>`/`]<REPOSITORY>]|-- [SUBPAGE] ]
```

This is to indicate that SUBPAGE is only allowed to be specified if the previous argument (or `--` in its place) was specified as well.