I don't know what version of hg you're using, but the histedit I've used drops me into an identical text editing setup as git rebase -i. It includes a summary of what the verbs mean in a comment at the bottom.
Interesting. The curses interface for histedit has been around since 2019, and I had no idea it wasn't the default since it's dramatically nicer to work with: https://www.mercurial-scm.org/relnotes/4.9