maintenance: fix synopsis in documentation

maintenance: fix synopsis in documentation

The synopsis for 'git maintenance' did not include the commands other
than the 'run' command. Update this to include the others. The 'start'
command is the only one of these that parses additional options, and
then only the --scheduler option.

Also move the 'register' command down after 'stop' and before
'unregister' for a logical grouping of the commands instead of an
alphabetical one. The diff makes it look as three other commands are
moved up.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>