docs: ext4.rst: document case-insensitive directories

Introduces the case-insensitive features on ext4 for system
administrators.  Explain the minimum of design decisions that are
important for sysadmins wanting to enable this feature.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>