job-scheduler: document minimum for period-ms

Since Android N, period-ms cannot be lower than 900.000 (15 minutes)

Author: xalexalex

scripts/termux-job-scheduler

@@ -12,7 +12,8 @@ show_usage () {
 	echo "Options for scheduling:"
 	echo "  --script text              path to the script to be called"
 	echo "  --job-id int               job id (will overwrite any previous job with the same id)"
-	echo "  --period-ms int            schedule job approximately every period-ms milliseconds (default 0 means once)"
+	echo "  --period-ms int            schedule job approximately every period-ms milliseconds (default 0 means once)."
+	echo "                             Note that since Android N, the minimum period is 900.000ms (15 minutes)."
 	echo "  --network text             run only when this type of network available (default any): any|unmetered|cellular|not_roaming|none"
 	echo "  --battery-not-low boolean  run only when battery is not low, default true (at least Android O)"
 	echo "  --storage-not-low boolean  run only when storage is not low, default false (at least Android O)"