notes: move the documentation to the struct

notes: move the documentation to the struct

Its better to document the struct members directly instead of on a
function that takes a pointer to the struct. This will also make it
easier to update the documentation in the future.

Make adjustments for this new context. Also drop “may contain” since we
don’t need to emphasize that a list could be empty.

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