[DOCFIX] Add the POSIX user group policy doc
[DOCFIX] Add the POSIX user group policy doc

### What changes are proposed in this pull request?

Add the POSIX user group policy doc

### Why are the changes needed?

Clear instruction of Alluxio POSIX API user group policy

### Does this PR introduce any user-facing changes?
NA

pr-link: Alluxio/alluxio#16449
change-id: cid-36b11b29ebda7af5788488d033883e01b21c7ebf
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