Add Init Rate setting info

[CapnBry]

The Init Rate option was removed for ExpressLRS 3.4, hold this PR for release https://github.com/ExpressLRS/ExpressLRS/pull/2424

I think people will come looking for it so I feel the docs should stay in the Lua section for at least one version. I've also added a description on two ways to save the Init Rate with the new system since it isn't fully automatic.