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>

Author: zhangyi089

Documentation/filesystems/ext4/journal.rst

@@ -260,8 +260,13 @@ which is 1024 bytes long:
      - s_num_fc_blocks
      - Number of fast commit blocks in the journal.
    * - 0x58
+     - __be32
+     - s_head
+     - Block number of the head (first unused block) of the journal, only
+       up-to-date when the journal is empty.
+   * - 0x5C
      - __u32
-     - s_padding[42]
+     - s_padding[40]
      -
    * - 0xFC
      - __be32