Add missing junitxml ini options to the reference docs

[nicoddemus]

No description provided.

[codecov]

Codecov Report

Merging #5338 into features will increase coverage by 0.9%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           features    #5338     +/-   ##
===========================================
+ Coverage      92.4%   93.31%   +0.9%     
===========================================
  Files           115      115             
  Lines         26294    26294             
  Branches       2597     2597             
===========================================
+ Hits          24297    24535    +238     
+ Misses         1677     1448    -229     
+ Partials        320      311      -9

Impacted Files	Coverage Δ
testing/test_pytester.py	54.04% <0%> (-34.98%)	⬇️
testing/test_reports.py	65.74% <0%> (-34.26%)	⬇️
testing/test_pluginmanager.py	49.57% <0%> (-25.43%)	⬇️
src/_pytest/pytester.py	86.11% <0%> (-3.4%)	⬇️
src/_pytest/reports.py	92.11% <0%> (-2.96%)	⬇️
testing/test_recwarn.py	97.03% <0%> (-2.55%)	⬇️
testing/test_tmpdir.py	97.31% <0%> (-1.62%)	⬇️
src/_pytest/config/__init__.py	93.79% <0%> (-0.64%)	⬇️
testing/test_pdb.py	47.55% <0%> (ø)	⬆️
src/_pytest/main.py	96.06% <0%> (+0.21%)	⬆️
... and 12 more

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[asottile]