doc: Clarify the purpose of pdm outdated and --unconstrained option

[frostming]

Signed-off-by: Frost Ming me@frostming.com

Pull Request Checklist

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

Related to #2962

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.30%. Comparing base (af4267b) to head (1bbddd0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2965   +/-   ##
=======================================
  Coverage   85.30%   85.30%           
=======================================
  Files         106      106           
  Lines       10697    10697           
  Branches     2337     2337           
=======================================
  Hits         9125     9125           
  Misses       1094     1094           
  Partials      478      478           

Flag	Coverage Δ
unittests	85.07% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

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