doc txt & -h consistency: use "[<label>...]" for "zero or more"

doc txt & -h consistency: use "[<label>...]" for "zero or more"

Correct uses of "<label>..." where we really meant to say
"[<label>...]", i.e. the command in question taken an optional set of
"<label>". As the CodingGuidelines notes "[o]ptional parts [should be]
enclosed in square brackets".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>