docs: document symlink restrictions for dot-files

docs: document symlink restrictions for dot-files

We stopped allowing symlinks for .gitmodules files in 10ecfa7649
(verify_path: disallow symlinks in .gitmodules, 2018-05-04), and we
stopped following symlinks for .gitattributes, .gitignore, and .mailmap
in the commits from 204333b015 (Merge branch 'jk/open-dotgitx-with-nofollow',
2021-03-22). The reasons are discussed in detail there, but we never
adjusted the documentation to let users know.

This hasn't been a big deal since the point is that such setups were
mildly broken and thought to be unusual anyway. But it certainly doesn't
hurt to be clear and explicit about it.

Suggested-by: Philip Oakley <philipoakley@iee.email>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>