Clarify transitivity of overrides and conflicts

[lolbinarycat]

there's no template for documentation issues, so i'm putting this here.

Arg::overrides_with notes that overriding is symmetric, but it's unclear if it is transitive.

if A conflicts with Band C, does B conflict with C?

for a command that has several modes (eg. install, update, uninstall), this could potentially cause subtle bugs where it is possible to make two modes enabled at once.

[epage]

If

• A conflicts with B
• A conflicts with C

I don't see why B would conflict with C

If

• A conflicts with B
• B conflicts with C

I don't see why A should conflict with C. Bs conflicts don't necessarily reflect on A and Cs relationship.

The main case where transitive does come up is with ArgGroup (if you conflict with an arg group, you confict with all members). We do have bugs where nested ArgGroups aren't correctly being calculated.