Improve Fuse stressbench doc
Improve Fuse stressbench doc

### What changes are proposed in this pull request?
Update the Fuse stressbench doc
### Why are the changes needed?
Give users clear ideas of what is fuse stressbench
### Does this PR introduce any user facing changes?
Doc

pr-link: Alluxio/alluxio#14201
change-id: cid-fad0cebde4a2e17476c578a6ddb994123a2d9c4f
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