gh stack add docs confusing about including commit

[jglick]

https://docs.github.com/en/pull-requests/reference/stacked-prs-cli-commands#gh-stack-add discussion of --message says

Create a commit with this message before creating the branch [emphasis added]

Yet suggested usages such as

gh stack add -Am "Add tests" test-layer

suggest the opposite, that the branch is created, and then the commit is applied to that branch (which would be more intuitive).