Fix #3080: multiline rubrics and citations are broken in texinfo builder

[tk0miya]

Feature or Bugfix

• Bugfix

Purpose

• refs: texinfo indentation bug? #3080

[codecov]

Codecov Report

Merging #5600 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #5600      +/-   ##
=========================================
+ Coverage   83.19%   83.2%   +0.01%     
=========================================
  Files         292     292              
  Lines       39193   39208      +15     
  Branches     5873    5873              
=========================================
+ Hits        32605   32623      +18     
+ Misses       5224    5221       -3     
  Partials     1364    1364

Impacted Files	Coverage Δ
sphinx/writers/texinfo.py	88.08% <100%> (+0.25%)	⬆️
tests/test_build_texinfo.py	86.66% <100%> (+4.31%)	⬆️
sphinx/builders/texinfo.py	74.14% <0%> (+0.68%)	⬆️

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