Add warning about pre-existing links placed in an SFTP chroot

This commit adds a note warning the user about placing pre-existing
links in a directory used as an SFTP chroot. While SFTP will prevent
creation of such links, someone with access to the filesystem on the
target host could place links in the chroot which point to outside
of it. This may be desirable in some cases, but it should be done
with caution.