doc: Mention a Cygwin 3.4.6 bug

[Bruno Haible]

readlinkat() has a bug on Cygwin, that's not worth working around. So I'm
just documenting it:


2023-04-18  Bruno Haible  <bruno@clisp.org>

        doc: Mention a Cygwin 3.4.6 bug.
        * doc/posix-functions/readlinkat.texi: Mention a readlinkat bug on
        Cygwin < 3.4.7.

diff --git a/doc/posix-functions/readlinkat.texi 
b/doc/posix-functions/readlinkat.texi
index 2cd2e7be5b..96ae7ec67a 100644
--- a/doc/posix-functions/readlinkat.texi
+++ b/doc/posix-functions/readlinkat.texi
@@ -55,6 +55,11 @@
 it sets @code{errno} to @code{EINVAL}:
 AIX 7.2.
 @item
+When this function is called on an empty file name, it fails with error
+@code{EBADF} instead of @code{ENOENT}:
+@c https://cygwin.com/pipermail/cygwin/2023-April/253510.html
+Cygwin 3.4.6.
+@item
 Symlink contents do not always have a trailing null byte, and there is
 no indication if symlink contents were truncated if the return value
 matches the length.  Furthermore,