x/review/git-codereview: package documentation for multi-commit branch workflow does not match behavior

The package documentation for git-codereview says:

The 'git codereview change' command amends the top commit in the stack (HEAD).

However, CL 20049 appears to have removed this behavior, instead stating that some combination git commit --amend and git rebase should be used instead.

I'd appreciate it if these docs were updated to better explain the current behavior (potentially with an example).

I'm working on a multi-change contribution, and am finding this a little bit confusing to figure out.