Documentation/git-rebase.txt: fix -f description to match

"Current branch is a descendant of the commit you are rebasing onto"
does not necessarily mean "rebase" requires "--force". Presence of
merge commit(s) makes "rebase" perform its default flattening actions
anyway.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>