docs: clarify API routing (and with --web.route-prefix)
docs: clarify API routing (and with --web.route-prefix)

Signed-off-by: Adam Shannon <1bf010ef9b5822ad9e26c3fbef055e410417f61d@gmail.com>

GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,prometheus/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager