Patches attached.

The reasoning behind the changes as follow:

1. `revert-buffer`: its current docs only mention that it tries its best to keep markers, but it does not mention that it may fail to do so, and that there is a slower but better alternative to this function. The lack of it being mentioned resulted in me researching its code down to C, and also another maintainer testing a bug, and yet another contributor writing up a large post about the new alternative revert functions…¹ All of that could've been avoided had `revert-function` referred to the new alternative, so that's what the patch does.
2. `auto-revert-mode`: Even with the change in 1 it's still unclear how to make use of that for `auto-revert-mode`. It does not even mention that the mode may break markers, so a person would have to debug it and find out revert-buffer, which in turn mentions about the problem, but doesn't really mention how to fix the auto-revert-mode specifically. So let's save people time, and just have a paragraph about it as well.

1: https://github.com/magit/magit/issues/4442