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