Mutual exclusivity of group should be shown in help (and not only in usage)

Mutually exclusive groups are shown as `[-a A | -b B]` in usage. When displaying help, there is no mention of the mutual exclusivity:

```
Group:
  The description.

  -a A
  -b B
```

There should also be an indication of the mutual exclusivity in the help text.

This must be behind a flag to preserve backward compatibility.