fix(doc): eliminate all the warnings of cargo doc --document-private-items

[lmatz]

What's changed and what's your intention?

PLEASE DO NOT LEAVE THIS EMPTY !!!
As the title.

Shall we now make cargo doc a part of CI and let it fail on warnings?

Refer to a related PR or issue link (optional)

#1437

[skyzh]

Shall we now make cargo doc a part of CI and let it fail on warnings?

cargo doc doesn't support fail on warning IIRC 🤣

[lmatz]

Let me investigate this a bit rust-lang/rust#79792

[codecov]

Codecov Report

Merging #1519 (0f40c85) into main (7d7a4b3) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #1519   +/-   ##
=========================================
  Coverage     70.45%   70.46%           
  Complexity     2766     2766           
=========================================
  Files          1031     1031           
  Lines         90539    90539           
  Branches       1790     1790           
=========================================
+ Hits          63793    63795    +2     
+ Misses        25855    25853    -2     
  Partials        891      891           

Flag	Coverage Δ
java	61.01% <ø> (ø)
rust	72.46% <ø> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
rust/meta/src/barrier/command.rs	53.44% <ø> (ø)
rust/meta/src/barrier/info.rs	100.00% <ø> (ø)
rust/meta/src/barrier/mod.rs	79.67% <ø> (ø)
rust/meta/src/stream/graph/stream_graph.rs	58.26% <ø> (ø)
rust/meta/src/stream/scheduler.rs	97.17% <ø> (ø)
rust/meta/src/stream/stream_manager.rs	75.72% <ø> (ø)
rust/stream/src/executor/lookup/impl_.rs	98.02% <ø> (ø)
rust/stream/src/executor/lookup/sides.rs	11.11% <ø> (ø)
rust/stream/src/executor_v2/error.rs	0.00% <ø> (ø)
rust/stream/src/lib.rs	100.00% <ø> (ø)
... and 1 more

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