Adopt refresh_delays as cache sources TTL

https://github.com/DNSCrypt/dnscrypt-proxy/pull/2334

config/dnscrypt-proxy.toml

@@ -697,9 +697,9 @@ allowed_ips_file = 'allowed-ips.txt'
 ## If the `urls` property is missing, cache files and valid signatures
 ## must already be present. This doesn't prevent these cache files from
 ## expiring after `refresh_delay` hours.
-## Cache freshness is checked every 24 hours, so values for 'refresh_delay'
-## of less than 24 hours will have no effect.
-## A maximum delay of 168 hours (1 week) is imposed to ensure cache freshness.
+## `refreshed_delay` must be in the [24..168] interval.
+## The minimum delay of 24 hours (1 day) avoids unnecessary requests to servers.
+## The maximum delay of 168 hours (1 week) ensures cache freshness.
 
 [sources]