Fix #5198: document not in toctree warning when including files only for parallel builds

[tk0miya]

Feature or Bugfix

• Bugfix

Purpose

• refs: document not in toctree warning when including files only for parallel builds #5198 (again)

[codecov]

Codecov Report

Merging #5313 into 1.7 will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##              1.7    #5313      +/-   ##
==========================================
+ Coverage   82.04%   82.04%   +<.01%     
==========================================
  Files         284      284              
  Lines       37932    37932              
  Branches     5880     5880              
==========================================
+ Hits        31120    31121       +1     
  Misses       5498     5498              
+ Partials     1314     1313       -1

Impacted Files	Coverage Δ
sphinx/environment/__init__.py	79.73% <0%> (ø)	⬆️
sphinx/builders/html.py	82.53% <0%> (+0.11%)	⬆️

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