doc: add some comments for keys & rename keys -> key

[xxchan]

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What's changed and what's your intention?

• See https://github.com/singularity-data/risingwave/pull/3700/files#diff-2b8f52f88a4b68d52744d240d51f436cc58b37b572fbf797c194e39edf91ed9b for the added comments
• rename keys -> key: a key is a whole, consisting of multiple columns. So we should call it key instead of keys

[codecov]

Codecov Report

Merging #3700 (dc8bf0c) into main (36e2f25) will decrease coverage by 0.01%.
The diff coverage is 78.07%.

@@            Coverage Diff             @@
##             main    #3700      +/-   ##
==========================================
- Coverage   74.32%   74.31%   -0.02%     
==========================================
  Files         792      792              
  Lines      111746   111746              
==========================================
- Hits        83052    83040      -12     
- Misses      28694    28706      +12     

Flag	Coverage Δ
rust	74.31% <78.07%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/batch/src/executor/expand.rs	52.94% <0.00%> (-4.03%)	⬇️
src/batch/src/executor/join/hash_join.rs	84.80% <ø> (ø)
src/batch/src/executor/join/sort_merge_join.rs	70.98% <0.00%> (ø)
src/batch/src/task/hash_shuffle_channel.rs	0.00% <0.00%> (ø)
src/ctl/src/cmd_impl/table/scan.rs	0.00% <0.00%> (ø)
src/frontend/src/catalog/pg_catalog/mod.rs	8.75% <ø> (ø)
...c/frontend/src/optimizer/plan_node/batch_expand.rs	0.00% <0.00%> (ø)
.../frontend/src/optimizer/plan_node/stream_expand.rs	0.00% <0.00%> (ø)
...rc/frontend/src/optimizer/plan_node/stream_topn.rs	54.71% <0.00%> (ø)
...ontend/src/stream_fragmenter/rewrite/delta_join.rs	0.00% <0.00%> (ø)
... and 48 more

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[xiangjinwu]

rename keys -> key: a key is a whole, consisting of multiple columns. So we should call it key instead of keys

At least the keys in expand's subset is not following this definition ... It was used as a synonym of "column index" ... I will take care of fixing the naming there.