Improve misleading passage about custom merge drivers

What does this MR do?

I am working with a customer (internal) who's interested in skipping certain files in a merge. They have found the documentation about custom merge drivers, which suggests that it can do exactly what they are looking for. This advice is misleading, because some types of merges do not invoke a custom merge driver at all; they are only invoked in case of a merge conflict that cannot be automatically resolved, so it will not be used in for example a fast-forward merge, or a merge where the file changed on only one of the branches.

There are many examples of people misunderstanding this Git feature in a similar way:

https://stackoverflow.com/questions/68643219/merge-drivers-dont-work-when-merging-into-default-branch https://www.reddit.com/r/git/comments/51jgu4/help_gitattributes_ignored_when_merging/ https://forum.gitlab.com/t/gitlab-enable-merge-driver-and-configure-gitattributes-exclude-file-folders-on-merge-request/8254

We shouldn't perpetuate this misconception.

Related issues

#18830 (closed)

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