asyncio docs: cover accidental bad reactor installation

[Gallaecio]

Resolves #5981.

Changes:

• Changed section sorting, let me know if it makes sense to you.
• Reworded the Windows section to avoid Python version references not necessary since the 3.7 drop, and to simplify things. I removed the instructions to manually changing the loop because I cannot think of a need for them given install_reactor, but I am not 100% sure.
• Added a section about handling early module-level Twisted imports that install the default, non-asyncio reactor. This was the initial goal here.

I wonder if we need to cover better the possibility of using a Twisted-powered software that installs a reactor before any user code is executed. I checked and scrapyd seems not to have an issue with that anymore, and I don’t know what other Twisted-powered stuff is popularly used to run Scrapy.

[codecov]

Codecov Report

Merging #6000 (692e0d6) into master (09c63a1) will not change coverage.
The diff coverage is n/a.

❗ Current head 692e0d6 differs from pull request most recent head 72de48b. Consider uploading reports for the commit 72de48b to get more accurate results

@@           Coverage Diff           @@
##           master    #6000   +/-   ##
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  Coverage   89.24%   89.24%           
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  Files         162      162           
  Lines       11325    11325           
  Branches     1835     1835           
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  Hits        10107    10107           
  Misses        927      927           
  Partials      291      291           

[wRAR]

Looks good to me.