docs(yaml): improve yaml document

[kt3k]

part of #3764

This PR documents parse and stringify of yaml package. I skipped documenting other items (such as Schema, Type, etc) as they are planned to be excluded from public APIs #5117

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.72%. Comparing base (a4d4e51) to head (28b487d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5127   +/-   ##
=======================================
  Coverage   92.71%   92.72%           
=======================================
  Files         480      480           
  Lines       38350    38351    +1     
  Branches     5400     5401    +1     
=======================================
+ Hits        35558    35561    +3     
+ Misses       2739     2734    -5     
- Partials       53       56    +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.