bug#49661: (patches) Improve docs for revert-buffer and auto-revert-mode

[Konstantin Kharlamov]

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

0001-Mention-that-revert-buffer-isn-t-best-for-markers-pr.patch
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0002-Mention-that-auto-revert-mode-may-break-markers.patch
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