expand help for --download option
expand help for --download option

Show the syntax for requesting a specific version of a patch.

Change-Id: Id72935c715a7b5c17722400dde6175b99e61860b
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack-infra/git-review