Git usage: improve guidance message, add examples.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
!fixup: Help: add 'guides' and 'commands' options

remove the separate 'commands' option, leaving 'all' for just that $gmane/215827

Signed-off-by: Philip Oakley <philipoakley@iee.org>