Resolve a regression when using newish automagic "write_image"
modes, by always padding to the end of affected sectors.

Also document some issues associated with those automagic options,
in the User's Guide and also some related code comments.

We might need similar padding at the *beginning* of some sectors,
but this is a minimalist fix for the problems which have currently
been reported (plus doc updates).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>