Ext4Pkg: Add documentation surrounding ext4 directory entries

Several questions have popped up regarding the ext4 directory entry
layout and contents off-list. Attempt to clarify these issues by adding
some explanatory comments.

Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Marvin Häuser <mhaeuser@posteo.de>
Reviewed-by: Marvin Häuser <mhaeuser@posteo.de>