DOC: Update docs on accessing callback arguments in errback

[dsandeep0138]

Update docs with the info on accessing callback arguments
(cb_kwargs) in errback. Also, included an example for the same.

Fixes #4598

[codecov]

Codecov Report

Merging #4634 into master will increase coverage by 0.03%.
The diff coverage is 91.25%.

@@            Coverage Diff             @@
##           master    #4634      +/-   ##
==========================================
+ Coverage   84.76%   84.80%   +0.03%     
==========================================
  Files         163      163              
  Lines        9978     9987       +9     
  Branches     1489     1487       -2     
==========================================
+ Hits         8458     8469      +11     
  Misses       1254     1254              
+ Partials      266      264       -2     

Impacted Files	Coverage Δ
scrapy/spiders/feed.py	66.15% <ø> (ø)
scrapy/commands/parse.py	20.21% <33.33%> (ø)
scrapy/contracts/default.py	84.61% <80.00%> (+0.30%)	⬆️
scrapy/core/scraper.py	87.97% <80.00%> (ø)
scrapy/shell.py	67.96% <80.00%> (-0.25%)	⬇️
scrapy/exporters.py	100.00% <100.00%> (ø)
scrapy/item.py	98.73% <100.00%> (ø)
scrapy/loader/__init__.py	95.54% <100.00%> (-0.03%)	⬇️
scrapy/pipelines/files.py	60.64% <100.00%> (+0.71%)	⬆️
scrapy/pipelines/images.py	91.81% <100.00%> (+1.16%)	⬆️
... and 2 more

[Gallaecio]

Nice! Thanks for going the extra mile and creating a section with an example, it’s really appreciated!

[elacuesta]

Thanks!

[dsandeep0138]

Thanks @Gallaecio and @elacuesta for the review!