ext4, doc: fix and improve directory hash tree description

Some of the details about how directory hash trees work were confusing or
outright wrong, this patch should fix those.

A note on dx_tail's dt_reserved member, as far as I can tell the kernel
never sets this explicitly, so its content is apparently left-overs from
what was there before (for the dx_root I've seen remnants of a
ext4_dir_entry_tail struct from when the dir was not yet a hash dir).

Signed-off-by: Zeno Endemann <zeno.endemann@mailbox.org>
Message-ID: <20250925152435.22749-1-zeno.endemann@mailbox.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>