doc: add comments to permutation prover structs

[guorong009]

Description

Document the permutation prover types, in halo2_backend

Related issues

Resolve #264

Changes

• add comments to CommittedSet, Committed and Evaluated structs in halo2_backend/plonk/permutation/prover.rs

[guorong009]

@davidnevadoc Can you take a look at the PR? It's a tiny PR for documentation. So, I think we can merge this before new release.

[guorong009]

Mark as draft until new release.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.99%. Comparing base (7c368af) to head (e817f27).

Additional details and impacted files

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##             main     #355   +/-   ##
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  Coverage   84.99%   84.99%           
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  Files          85       85           
  Lines       18684    18684           
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  Hits        15880    15880           
  Misses       2804     2804           

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[guorong009]

Doc looks good but I find the struct names somewhat confusing (not changed in this PR). In particular, Committed being a Vec of CommittedSet and not the other way around seems very counter intuitive. Thoughts on this @adria0 @guorong009 ? Other than that, LGTM 👍

Yes, they are very counter intuitive. How about renaming the CommittedSet? eg. PermPolyCommitted, SinglePolyCommitted, CommittedSingle