patman: Properly document the patchwork_url setting

The "Series-patchwork-url:" tag description says that it overrides the
settings file but doesn't specify the name of the setting. Elsewhere
in the documentation about the "useful" settings we see a setting that
sounds promising called "patchwork_server" that's actually not a valid
setting. It should be "patchwork_url".

Fix these problems so the doc is right and more useful.

Signed-off-by: Douglas Anderson <dianders@chromium.org>