ext4: update doc about journal superblock description

Update the description in doc about the s_head parameter in the journal
superblock.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://lore.kernel.org/r/20230322013353.1843306-4-yi.zhang@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>