Document (or fix) `set-upstream` behavior for stacks

The following discussion from !1687 (merged) should be addressed:

• @hacks4oats started a discussion: (+1 comment)

  question: Should we document this behavior? I don't know how often this happens, but it's possible that a user may have a fork of a repository, and would like to make the merge request against their own fork instead of the forked repo. This might catch them by surprise if it's not documented.

Implementation plan

• Document the behavior mentioned in this comment
• A likely spot would be in the sync help: https://gitlab.com/gitlab-org/cli/blob/main/commands/stack/sync/stack_sync.go#L64