docs: document core public API and guard it with deny(missing_docs)

[sshaplygin]

Problem

Building with -W missing_docs reports roughly 470 undocumented public items, spread across most modules. docs.rs therefore shows bare signatures for types users touch on their first query.

Change

Start with the core surface and make it non-regressable:

• errors.rs — YdbOrCustomerError::to_ydb_error, the five YdbIssueSeverity variants, the YdbIssue issue_code/message fields.
• result.rs — ResultSet and its rows/is_truncated, Row and both remove_field* methods, ResultSetRowsIter.
• pub_traits.rs — TokenInfo, Credentials and both trait methods. The existing implementation notes on create_token (caching, background renewal, behaviour while renewal fails) were plain // comments invisible on docs.rs; they are now rustdoc, with the originals kept in place.
• waiter.rs — Waiter and Waiter::wait.

Then #[deny(missing_docs)] on those four module declarations in lib.rs, so the documented surface cannot regress.

The other modules stay unguarded and can be converted one at a time by the same pattern — the largest remaining are client_topic/list_types.rs (72 items), client_topic/client.rs (46) and table_requests.rs (41).

Verification

• -W missing_docs reports 0 remaining warnings in the four modules.
• Confirmed the guard bites: deleting one doc comment turns the build red with error: missing documentation for a method.
• All new intra-doc links resolve. cargo doc reports 17 broken links crate-wide, all pre-existing and all in other modules (10 in client_query/builders.rs).
• cargo fmt --check, clippy -D warnings, cargo test --workspace and cargo test --doc all pass.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.85%. Comparing base (a6d7911) to head (78cf418).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #586      +/-   ##
==========================================
- Coverage   86.91%   86.85%   -0.06%     
==========================================
  Files         198      198              
  Lines       19492    19492              
==========================================
- Hits        16941    16930      -11     
- Misses       2551     2562      +11     

Flag	Coverage Δ
rust-1.88.0	86.85% <ø> (-0.06%)	⬇️
rust-1.96.1	87.15% <ø> (-0.01%)	⬇️
tests	86.85% <ø> (-0.06%)	⬇️
ubuntu	86.85% <ø> (-0.06%)	⬇️

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[rekby]

Тут непонятно различие между этими двумя ошибками. И в fatal и в error - операция дальше не продолжается.

[rekby]

На эти коды тоже нельзя полагаться в общем случаи. Они более стабильны, чем текст. Но гарантии их стабильности и неизменности на конкретных сценариях нет. Это низкоуровневые штуки, больше для диагностики. И общая рекомендация - не завязываться на них.