NEWS: document possible breakage from saved-search format change

If the user has unsorted or alphabetically sorted saved-searches these
should continue to work. If they have some custom lisp sort function
then it will need updating to work with the new saved-sort
format. Document this, and how the updating should be done.

Also roll in fixes for the markdown in the first part of the NEWS
suggested by Tomi: change `just work' to *just work* and removed
periods from the end of subtitle lines.