Suggest faster ping in `PeerManager::timer_tick_occurred` docs

Suggest faster ping in `PeerManager::timer_tick_occurred` docs

This clarifies the docs for `PeerManager::timer_tick_occurred` to
note that the call rate is entirely up to the user, and also
suggests a faster ping rate of "once every five to ten seconds"
instead of "every 30 seconds". There isn't a lot of reason to want
to ping less often, and faster ping means we detect disconnects
sooner, which is important.