Move all notes and warnings to description text.
Refine some small clarification.
Do not use NOTE/WARNING unless there is a serious reason (data loss).
With LUKS2 crypt_keyslot_change_by_passphrase() call
does not have to overwrite binary keyslot
area in-place when user asked for specific keyslot id.
If there's enough free space in keyslot binary area
we can write new keyslot material in the the free area
(identified temporarily by new keyslot id) and switch
pointers (json metadata) to point to the new keyslot area after
the keyslot area write is complete. The old keyslot
area gets deleted after the new area write is finished.
Otherwise we needlesly risk to lose the existing keyslot
if the operation gets interupted.
With this patch LUKS2 crypt_keyslot_change_by_passphrase()
overwrites existing keyslot (including keyslot area)
only if there's no free space and therefore in-place update
is necessary.
Fixes: #839.
