ext4: Update Documentation/filesystems/ext4.txt

Since Ext4 is supposed to be stable in 2.6.28-rc, ext4's documentation
file should be updated.

[ More updates also added by Theodore Ts'o. ]

Signed-off-by: Diego Calleja <diegocg@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>