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>

Documentation/gitattributes.txt

@@ -1247,6 +1247,12 @@ to:
 [attr]binary -diff -merge -text
 ------------
 
+NOTES
+-----
+
+Git does not follow symbolic links when accessing a `.gitattributes`
+file in the working tree. This keeps behavior consistent when the file
+is accessed from the index or a tree versus from the filesystem.
 
 EXAMPLES
 --------

Documentation/gitignore.txt

@@ -149,6 +149,10 @@ not tracked by Git remain untracked.
 To stop tracking a file that is currently tracked, use
 'git rm --cached'.
 
+Git does not follow symbolic links when accessing a `.gitignore` file in
+the working tree. This keeps behavior consistent when the file is
+accessed from the index or a tree versus from the filesystem.
+
 EXAMPLES
 --------
 

Documentation/gitmailmap.txt

@@ -55,6 +55,13 @@ this would also match the 'Commit Name <commit&#64;email.xx>' above:
 	Proper Name <proper@email.xx> CoMmIt NaMe <CoMmIt@EmAiL.xX>
 --
 
+NOTES
+-----
+
+Git does not follow symbolic links when accessing a `.mailmap` file in
+the working tree. This keeps behavior consistent when the file is
+accessed from the index or a tree versus from the filesystem.
+
 EXAMPLES
 --------
 

Documentation/gitmodules.txt

@@ -98,6 +98,14 @@ submodule.<name>.shallow::
 	shallow clone (with a history depth of 1) unless the user explicitly
 	asks for a non-shallow clone.
 
+NOTES
+-----
+
+Git does not allow the `.gitmodules` file within a working tree to be a
+symbolic link, and will refuse to check out such a tree entry. This
+keeps behavior consistent when the file is accessed from the index or a
+tree versus from the filesystem, and helps Git reliably enforce security
+checks of the file contents.
 
 EXAMPLES
 --------