cleanup: enable missing_docs by default

[suzmue]

Alternatively, we document these things, but that can be done as follow up work. Cargo.toml does not allow inheriting some lints from the workspace and overriding others, so we have to put the annotations in the code.

For #5459

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.75%. Comparing base (1dd0704) to head (2ff4ce5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5536   +/-   ##
=======================================
  Coverage   97.75%   97.75%           
=======================================
  Files         218      218           
  Lines       49498    49498           
=======================================
  Hits        48389    48389           
  Misses       1109     1109           

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