Add comment to highlight the default prod value for minimumFetchIntervalMillis.

Issue addressed: firebase/firebase-js-sdk#2841

Author: Athira M

snippets/remoteconfig-next/index/rc_set_minimum_fetch_time.js

@@ -5,5 +5,6 @@
 // 'npm run snippets'.
 
 // [START rc_set_minimum_fetch_time_modular]
+// The default and recommended production fetch interval for Remote Config is 12 hours.
 remoteConfig.settings.minimumFetchIntervalMillis = 3600000;
 // [END rc_set_minimum_fetch_time_modular]