notes: update documentation for `use_default_notes`

notes: update documentation for `use_default_notes`

`suppress_default_notes` was renamed to `use_default_notes` in
3a03cf6b1d (notes: refactor display notes default handling,
2011-03-29).

The commit message says that “values less than one [indicates] “not
set” ”, but what was meant was probably “less than zero” (the author of
3a03cf6b1d agrees on this point).

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>