ext4: update on-disk format documentation for fs-verity

Document the format of verity files on ext4, and the corresponding inode
and superblock flags.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>

Documentation/filesystems/ext4/overview.rst

@@ -24,3 +24,4 @@ order.
 .. include:: bigalloc.rst
 .. include:: inlinedata.rst
 .. include:: eainode.rst
+.. include:: verity.rst