Add a comment about tmpname + '^'.

[wRAR]

I had no idea why is this needed and how should it be used (https://github.com/scrapy/scrapy/pull/5285/files#r844017508, https://github.com/scrapy/scrapy/pull/5682/files/726680c7125ab3a6622b12e25d45dbfedc5a39b3#r1027288466) so I tracked it down to 3012030 and added this comment.

[codecov]

Codecov Report

Merging #5736 (face18b) into master (0417e3e) will decrease coverage by 0.18%.
The diff coverage is n/a.

❗ Current head face18b differs from pull request most recent head f85c3f3. Consider uploading reports for the commit f85c3f3 to get more accurate results

@@            Coverage Diff             @@
##           master    #5736      +/-   ##
==========================================
- Coverage   88.95%   88.77%   -0.19%     
==========================================
  Files         162      162              
  Lines       10938    10931       -7     
  Branches     1797     1620     -177     
==========================================
- Hits         9730     9704      -26     
- Misses        927      943      +16     
- Partials      281      284       +3     

Impacted Files	Coverage Δ
scrapy/utils/conf.py	92.10% <ø> (ø)
scrapy/core/downloader/handlers/__init__.py	83.63% <0.00%> (-9.10%)	⬇️
scrapy/utils/test.py	60.27% <0.00%> (-5.48%)	⬇️
scrapy/squeues.py	95.34% <0.00%> (-4.66%)	⬇️
scrapy/downloadermiddlewares/cookies.py	95.78% <0.00%> (-2.11%)	⬇️
scrapy/pipelines/images.py	95.58% <0.00%> (-1.48%)	⬇️
scrapy/shell.py	67.96% <0.00%> (-0.79%)	⬇️
scrapy/core/http2/protocol.py	82.91% <0.00%> (-0.51%)	⬇️
scrapy/utils/python.py	90.90% <0.00%> (-0.06%)	⬇️
scrapy/utils/defer.py	97.29% <0.00%> (-0.04%)	⬇️
... and 5 more

[Gallaecio]

🎉