ext4: Add atomic block write documentation

Add an initial documentation around atomic writes support in ext4.

Acked-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Link: https://patch.msgid.link/d3893b9f5ad70317abae72046e81e4c180af91bf.1747337952.git.ritesh.list@gmail.com


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