[DOCFIX] Improve remote-logging doc
[DOCFIX] Improve remote-logging doc

Besides some small wording improvements, this adds 2 corrections:
1. BOTH `ALLUXIO_LOGSERVER_HOSTNAME` and `ALLUXIO_LOGSERVER_PORT` MUST
be set to make the remote logger active
2. The client remote logger needs some more params to work

pr-link: Alluxio/alluxio#12888
change-id: cid-87701c6bdae8493d052469ca86c617d1e8bc72ab
Alluxio/alluxio,Alluxio/alluxio,wwjiang007/alluxio,calvinjia/tachyon,wwjiang007/alluxio,bf8086/alluxio,Alluxio/alluxio,wwjiang007/alluxio,bf8086/alluxio,maobaolong/alluxio,maobaolong/alluxio,wwjiang007/alluxio,calvinjia/tachyon,maobaolong/alluxio,Alluxio/alluxio,bf8086/alluxio,maobaolong/alluxio,wwjiang007/alluxio,calvinjia/tachyon,maobaolong/alluxio,calvinjia/tachyon,calvinjia/tachyon,Alluxio/alluxio,wwjiang007/alluxio,Alluxio/alluxio,wwjiang007/alluxio,maobaolong/alluxio,Alluxio/alluxio,Alluxio/alluxio,bf8086/alluxio,wwjiang007/alluxio,wwjiang007/alluxio,Alluxio/alluxio,calvinjia/tachyon,calvinjia/tachyon,maobaolong/alluxio,bf8086/alluxio,bf8086/alluxio,bf8086/alluxio,Alluxio/alluxio,maobaolong/alluxio,maobaolong/alluxio,wwjiang007/alluxio,calvinjia/tachyon,bf8086/alluxio,maobaolong/alluxio