added faq for starting/stopping bsc node as a service (bnb-chain#264)

docs/BSC-FAQs-general.md

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+
+### How to gracefully shut down BSC node when running as a system service?
+
+Basically, bsc node can be shutdown gracefully, it could take around 15 seconds in general. But sometimes, some nodes failed to graceful shutdown due to the incorrect use of this flag: `--history.transactions 0`, if you just upgrade from `v1.2.x` to `v1.3.x`, please notice that `--txlookuplimit` has been replaced by `--history.transactions` since `v1.3.x`. [Reference](https://github.com/bnb-chain/bsc/issues/2163#issuecomment-1897652226)  
+
+Here is the service file and script that can be used to start/stop bsc node, for your reference. 
+
+`bsc.service`
+```
+[Unit]
+Description=bsc
+After=network.target
+
+[Service]
+Type=simple
+User=root
+Group=root
+ExecStart=/server/node/chaind.sh -start
+ExecReload=/server/node/chaind.sh -restart
+ExecStop=/server/node/chaind.sh -stop
+PrivateTmp=true
+Restart=always
+LimitNOFILE=500000
+RestartSec=5
+TimeoutStopSec=120
+StartLimitInterval=0
+
+[Install]
+```
+
+And `/server/node/chaind.sh` file:
+```
+#!/bin/bash
+export GOGC=100
+
+function startChaind() {
+    workspace=/server/node
+    ${workspace}/bsc --cache 18000 --history.transactions 0  --datadir ${workspace}/  --config ${workspace}/config.toml >> ${workspace}/bscnode.log 2>&1
+}
+
+function stopChaind() {
+    pid=`ps -ef | grep /server/node/bsc | grep -v grep | awk '{print $2}'`
+    if [ -n "$pid" ]; then
+        kill -TERM $pid
+        for((i=1;i<=40;i++));
+        do
+            pid=`ps -ef | grep /server/node/bsc | grep -v grep | awk '{print $2}'`
+            if [ -z "$pid" ]; then
+                break
+            fi
+            sleep 10
+        done
+    fi
+}
+
+CMD=$1
+
+case $CMD in
+-start)
+    echo "start"
+    startChaind
+    ;;
+-stop)
+    echo "stop"
+    stopChaind
+    ;;
+-restart)
+    stopChaind
+    sleep 3
+    startChaind
+    ;;
+*)
+    echo "Usage: chaind.sh -start | -stop | -restart .Or use systemctl start | stop | restart bsc.service "
+    ;;
+esac
+```