Added documentation for Spider.update_settings

[jxlil]

Added documentation for the Spider.update_settings class method.

Fixes #5745, closes #5757, closes #5784.

[codecov]

Codecov Report

Merging #5846 (bcffe38) into master (8aca47e) will increase coverage by 0.26%.
Report is 300 commits behind head on master.
The diff coverage is 92.88%.

❗ Current head bcffe38 differs from pull request most recent head af1be83. Consider uploading reports for the commit af1be83 to get more accurate results

@@            Coverage Diff             @@
##           master    #5846      +/-   ##
==========================================
+ Coverage   88.96%   89.23%   +0.26%     
==========================================
  Files         162      163       +1     
  Lines       11021    11370     +349     
  Branches     1797     1844      +47     
==========================================
+ Hits         9805    10146     +341     
+ Misses        938      930       -8     
- Partials      278      294      +16     

Files Changed	Coverage Δ
scrapy/__init__.py	84.21% <0.00%> (ø)
scrapy/downloadermiddlewares/httpcache.py	93.97% <ø> (ø)
scrapy/mail.py	77.77% <0.00%> (-0.88%)	⬇️
scrapy/commands/parse.py	29.26% <42.10%> (+1.07%)	⬆️
scrapy/utils/engine.py	77.27% <71.42%> (-6.94%)	⬇️
scrapy/pipelines/files.py	79.33% <75.00%> (+7.42%)	⬆️
scrapy/cmdline.py	68.70% <80.00%> (+0.73%)	⬆️
scrapy/signalmanager.py	81.81% <83.33%> (+1.81%)	⬆️
scrapy/addons.py	84.00% <84.00%> (ø)
scrapy/utils/display.py	93.10% <85.71%> (+0.24%)	⬆️
... and 54 more

... and 2 files with indirect coverage changes

[Gallaecio]

Besides my points below, I think it may be good to highlight the benefits of update_settings over custom_settings. I‘m thinking that the main one is that you can dynamically add, remove or change settings based on spider arguments and other settings that may be defined outside code (e.g. Scrapy Cloud settings), not sure if there are others.

[jxlil]

Done. The requested modifications were made and the last paragraph was replaced to highlight the benefits of update_settings()

[wRAR]

It doesn't cover everything that I want to be covered here, I'll try to suggest some additional wording.

[wRAR]

Thanks!