luci-app-statistics: Adjust ping graphs to show target hosts separately

Ping statistics plugin allows the user to define multiple ping targets.
The graphs have so far shown multiple hosts in a stacked way.
Adjust graphs to show the target hosts separately (overlay=true).

Clarify the plugin definition with some additional line spacing.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>