This comment has been out of date since notmuch 0.32. Although it
isn't really possible to explain all the options here, explain both
one new "split" way of doing things and the traditional one with
database inside $MAIL_ROOT/.notmuch.
This matches the heuristic used by "notmuch config set" to decide if
something is a list.
This change fixes the bug reported at [1].
[1]: id:6O3LTUhoXlrnkPWCtPJCP4cagU7mFVEdyTpcC_37BoSzStlARXDBa7oczy6hB0jyjGjBQvgj_jFV58cw0aNx-jUg1h1O-FQ7820k68C0X4M=@protonmail.com