[DOCFIX] Add additional requirement for fuse metrics
[DOCFIX] Add additional requirement for fuse metrics

To check standalone Fuse metrics, `alluxio.fuse.web.enabled` needs to be
set to `true` in `alluxio-site.properties`. Add this to the Posix-api
doc.

pr-link: Alluxio/alluxio#14436
change-id: cid-1e3a4773d3030a29f2e98a119e8f7f9c6815d834
maobaolong/alluxio,maobaolong/alluxio,Alluxio/alluxio,Alluxio/alluxio,wwjiang007/alluxio,Alluxio/alluxio,Alluxio/alluxio,maobaolong/alluxio,Alluxio/alluxio,wwjiang007/alluxio,Alluxio/alluxio,wwjiang007/alluxio,maobaolong/alluxio,Alluxio/alluxio,wwjiang007/alluxio,Alluxio/alluxio,Alluxio/alluxio,maobaolong/alluxio,maobaolong/alluxio,wwjiang007/alluxio,wwjiang007/alluxio,maobaolong/alluxio,wwjiang007/alluxio,wwjiang007/alluxio,maobaolong/alluxio,maobaolong/alluxio,Alluxio/alluxio,wwjiang007/alluxio,wwjiang007/alluxio,maobaolong/alluxio