[Feature] Help text or title option for mutually-exclusive groups.

[emcd]

I am quite happy with the mutually-exclusive groups feature that you recently added. But, to prove that no good deed feature goes unpunished, I have a request: a help or title argument. Currently, this is hardcoded to At most one argument can overridden.. Aside from the fact that there is a missing be in that sentence, I would like to be able to better provide "titles" for the groups if they cannot live as subgroups within the groups for their corresponding owners.

In the above, I would like for each subgroup to live within its corresponding group: inscription options and display options. But, failing that, I would like to be able to change At most one argument can [be] overridden. to inscription options: target (choose one) and display options: target (choose one) or change mutually exclusive to inscription options: target and display options: target.

I can look into making a PR for this, but wanted to get your take first.