Add a message on how to make log refresh immediately when starting a …

…component (apache#6078)

### Motivation

Some users may confuse by pulsar/bookie log without flushing immediately.

### Modifications

Add a message in `bin/pulsar-daemon` when starting a component.

bin/pulsar-daemon

@@ -151,6 +151,7 @@ case $startStop in
 
     rotate_out_log $out
     echo starting $command, logging to $logfile
+    echo Note: Set immediateFlush to true in conf/log4j2.yaml will guarantee the logging event is flushing to disk immediately. The default behavior is switched off due to performance considerations.
     pulsar=$PULSAR_HOME/bin/pulsar
     nohup $pulsar $command "$@" > "$out" 2>&1 < /dev/null &
     echo $! > $pid