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

decouple rest api and java api

### Why are the changes needed?

Rest API deserve its own section

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

Eng document

pr-link: Alluxio/alluxio#16326
change-id: cid-02dc35ca9c5b5b9c08c8a2279f02ec6c49c7887f