docs(bigquery): improve RowIterator related docs

[alvarowolfx]

Towards #5844

[gemini-code-assist]

Code Review

This pull request significantly improves the documentation of the BigQuery query module by adding detailed descriptions and usage examples for the FromSql trait, as well as the Interval, Range, RowIterator, and Row structs. The review feedback correctly identifies several instances where private module paths (such as crate::query::CompleteQuery, crate::query::Row, and crate::query::RowIterator) are referenced in public documentation. These unresolved intra-doc links will trigger rustdoc::private_intra_doc_links warnings and fail CI builds, so they should be updated to use their public re-exported paths.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.26%. Comparing base (4ff2683) to head (3e33c3d).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6320   +/-   ##
=======================================
  Coverage   96.26%   96.26%           
=======================================
  Files         282      282           
  Lines       73304    73304           
=======================================
+ Hits        70563    70567    +4     
+ Misses       2741     2737    -4     

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