Removed the ability to run the -O and -t command line option together (#5516)

[MagnusOffermanns]

Fix for Issue 5516

If -t and -O are used together scrapy now throws a usageError. I also created a error message including examples.

Furthermore, I added an extended better deprecation warning in case -t and -o
are used together including two examples.

[MagnusOffermanns]

I have two inputs that I would like to get feedback on, as I do not really know how to handle the two cases. I would be very glad for your feedback.

1. The warnings and error messages I created are all in one line similar to the already existing ones. Would it not make sense to make them a little bit more readable by giving them new lines? Is it maybe wanted that they only have one line in order to display them better?
2. When the combination of -t and -O throws an error would it not also make sense to throw an error when -t and -o are used? Therefore making the -t option obsolete? How long do we have to keep -t till we can remove it from the source code?

I thank you for answering my questions and hope I could resolve this issue ok 🚀

[MagnusOffermanns]

I also extended the documentation for crawl and the help message. Maybe I also closed issue #5530.

With one caviat:
I was not able to make the -a command run. Therefore I did not add an example there. If someone gives me a tip how to use the -a option I will quickly add it as well.

Anyway, thank you for your help and feedback.

[wRAR]

Looks good but there are some typos, can you please fix those?

[codecov]

Codecov Report

Merging #5605 (65bc100) into master (9fbd819) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

❗ Current head 65bc100 differs from pull request most recent head 440d383. Consider uploading reports for the commit 440d383 to get more accurate results

@@            Coverage Diff             @@
##           master    #5605      +/-   ##
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- Coverage   88.67%   88.63%   -0.04%     
==========================================
  Files         162      162              
  Lines       10998    11000       +2     
  Branches     1801     1802       +1     
==========================================
- Hits         9752     9750       -2     
- Misses        966      968       +2     
- Partials      280      282       +2     

Impacted Files	Coverage Δ
scrapy/commands/__init__.py	73.25% <ø> (ø)
scrapy/utils/conf.py	92.10% <0.00%> (-1.65%)	⬇️
scrapy/core/downloader/__init__.py	90.97% <0.00%> (-1.51%)	⬇️