Better flag documentation.

[tvalentyn]

• improves documentation for a missing pipeline option:

Drive by changes:

• Remove flag modifications in apply calls: Adding experiments is now also done in run_pipeline . apply() seems like a wrong place to do this.
• Cleanup now uncecessary method in DF runner.

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[tvalentyn]

Run Python 3.9 PostCommit

[codecov]

Codecov Report

Merging #28753 (ab4e352) into master (8031162) will decrease coverage by 0.02%.
Report is 10 commits behind head on master.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #28753      +/-   ##
==========================================
- Coverage   72.25%   72.23%   -0.02%     
==========================================
  Files         684      684              
  Lines      101241   101236       -5     
==========================================
- Hits        73147    73125      -22     
- Misses      26516    26533      +17     
  Partials     1578     1578              

Flag	Coverage Δ
python	82.76% <ø> (+<0.01%)	⬆️

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

Files	Coverage Δ
...on/apache_beam/runners/dataflow/dataflow_runner.py	80.90% <ø> (+0.01%)	⬆️

... and 6 files with indirect coverage changes

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