docs(pd): update test commands and improve documentation clarity

[koi2000]

Purpose of the PR

• close #xxx

Main Changes

update PD-related documentation. update test commands and improve documentation clarity

Verifying these changes

• Trivial rework / code cleanup without any test coverage. (No Need)
• Already covered by existing tests, such as (please modify tests here).
• Need tests and can be verified as follows:
  • xxx

Does this PR potentially affect the following parts?

• Dependencies (add/update license info & regenerate_known_dependencies.sh)
• Modify configurations
• The public API
• Other affects (typed here)
• Nope

Documentation Status

• Doc - TODO
• Doc - Done
• Doc - No Need

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 1.58%. Comparing base (5b3d295) to head (61fbbb3).
⚠️ Report is 2 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (5b3d295) and HEAD (61fbbb3). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (5b3d295)	HEAD (61fbbb3)
	3	2

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #2893       +/-   ##
============================================
- Coverage     35.57%   1.58%   -34.00%     
+ Complexity      333      43      -290     
============================================
  Files           801     779       -22     
  Lines         67422   64907     -2515     
  Branches       8750    8302      -448     
============================================
- Hits          23985    1026    -22959     
- Misses        40886   63796    +22910     
+ Partials       2551      85     -2466     

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