man: document XFS_XFLAG_APPEND behavior for directories

man: document XFS_XFLAG_APPEND behavior for directories

For directories, the APPEND flag means that files cannot be unlinked
from the directory.  Files can be linked in or created, just not
unlinked.  Document this behavior, since it's been in the VFS for years
though not explicitly mentioned.  This patch is in preparation for
trying to hoist the fsgetxattr ioctl documentation to the man-pages
project.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>