Extend Request.meta documentation

[Gallaecio]

While reviewing #4141, I realized the existing Request.meta documentation could use some more information.

[codecov]

Codecov Report

Merging #5565 (f8709a1) into master (fa690fb) will increase coverage by 0.15%.
The diff coverage is n/a.

❗ Current head f8709a1 differs from pull request most recent head e76fd8b. Consider uploading reports for the commit e76fd8b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5565      +/-   ##
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+ Coverage   88.59%   88.75%   +0.15%     
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  Files         159      162       +3     
  Lines       11582    10774     -808     
  Branches     1885     1843      -42     
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- Hits        10261     9562     -699     
+ Misses        994      939      -55     
+ Partials      327      273      -54     

see 156 files with indirect coverage changes

[kmike]

Hm, I think cb_kwargs also works ok in this case - you can pass something like partial_data dict or list as a kwarg to all callbacks

[Gallaecio]

The use case I am trying to describe here is where you pass some specific metadata key to almost every callback, and new future callbacks are likely to get it as well, but often most callbacks only pass it over to the next callback. So, for example, start_url if you want to keep track of the root URL that originated every callback.

In those cases, adding the same parameter to every callback, specially to callbacks whose only function is to pass the parameter over to the next callback, may be too verbose, and using meta instead (and eventually STICKY_META_KEYS, #4141, which is what inspired this paragraph) may be better.

I do need to come up with a better wording. Let me give it at least 1 more try.

[kmike]

👍 I left a comment, but feel free to merge without addressing it.