doc: fix note about output capturing with pdb

[blueyed]

[skip travis]

[codecov]

Codecov Report

Merging #5004 into features will decrease coverage by 2.31%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           features    #5004      +/-   ##
============================================
- Coverage     95.97%   93.66%   -2.32%     
============================================
  Files           113      113              
  Lines         25359    25359              
  Branches       2501     2501              
============================================
- Hits          24339    23753     -586     
- Misses          714     1275     +561     
- Partials        306      331      +25

Impacted Files	Coverage Δ
testing/test_pdb.py	49.09% <0%> (-50.01%)	⬇️
testing/test_argcomplete.py	20.28% <0%> (-47.83%)	⬇️
src/_pytest/_argcomplete.py	33.33% <0%> (-41.67%)	⬇️
src/_pytest/unittest.py	76.19% <0%> (-17.99%)	⬇️
src/_pytest/debugging.py	65.08% <0%> (-16.57%)	⬇️
testing/test_unittest.py	85.58% <0%> (-13.52%)	⬇️
testing/test_conftest.py	90.33% <0%> (-9.3%)	⬇️
testing/test_pathlib.py	91.17% <0%> (-8.83%)	⬇️
src/_pytest/pytester.py	84.94% <0%> (-6.15%)	⬇️
testing/test_collection.py	95.23% <0%> (-4.58%)	⬇️
... and 20 more

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