checkout: document subtlety around --ours/--theirs

checkout: document subtlety around --ours/--theirs

During a 'rebase' (hence 'pull --rebase'), --ours/--theirs may
appear to be swapped to those who are not aware of the fact that
they are temporarily playing the role of the keeper of the more
authoritative history.

Add a note to clarify.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Simon A. Eugster <simon.eugster@eps.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>