Split up the --[no-]merged documentation into documentation that
documents each option independently. This is in line with how "branch"
and "for-each-ref" are documented, and makes subsequent changes to
discuss the limits & caveats of each option easier to read.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>