Documentation/git-rebase.txt: <upstream> must be given to specify <branch>

Current syntax description makes one wonder if there is any
syntactic way to distinguish between <branch> and <upstream> so that
one can specify <branch> but not <upstream>, but that is not the
case.

Make it explicit that these arguments are positional, i.e. the
earlier ones cannot be omitted if you want to give later ones.

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