Add the required indentation to fix this docs build error:

  Documentation/admin-guide/device-mapper/dm-crypt.rst:167: ERROR: Unexpected indentation.

Also split the documentation for read and write into separate blocks.

Signed-off-by: Shibu kumar shibukumar.bit@gmail.com
[jc: rewrote changelog]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240803183306.32425-1-shibukumar.bit@gmail.com