[PATCH 4 of 7 v2] histedit: explain basics of histedit commands

[timeless]

Augie Fackler wrote:
> It's a bit of a bummer to list the commands inline here hard-coded,
> because now it duplicates a bunch of text and if someone adds a rule
> in a third-party extension we're going to have a bad time. Can we do
> this at least somewhat dynamically somehow?

With the histedit @action stuff, if we could define hooks the way we
do for help revsets, perhaps...